Determinants and impact of calcium oxalate crystal deposition on renal outcomes in acute kidney injury patients

Abstract

Objectives

Calcium oxalate (CaOx) crystal deposition in acute kidney injury (AKI) patients is under recognized but impacts renal outcomes. This study investigates its determinants and effects.

Methods

We studied 814 AKI patients with native kidney biopsies from 2011 to 2020, identifying CaOx crystal deposition severity (mild: <5, moderate: 5–10, severe: >10 crystals per section). We assessed factors like urinary oxalate, citrate, urate, electrolytes, pH, tubular calcification index, and SLC26A6 expression, comparing them with creatinine-matched AKI controls without oxalosis. We analyzed how these factors relate to CaOx severity and their impact on renal recovery (eGFR < 15 mL/min/1.73 m² at 3-month follow-up). 

Results

CaOx crystal deposition was found in 3.9% of the AKI cohort (32 cases), with 72% due to nephrotoxic medication-induced tubulointerstitial nephritis. Diuretic use, higher urinary oxalate-to-citrate ratio induced by hypocitraturia, and tubular calcification index were significant contributors to moderate and/or severe CaOx deposition. Poor baseline renal function, low urinary chloride, high uric acid and urea nitrogen, tubular SLC26A6 overexpression, and glomerular sclerosis were also associated with moderate-to-severe CaOx deposition. Kidney recovery was delayed, with 43.8%, 31.2%, and 18.8% of patients having eGFR < 15 mL/min/1.73 m² at 4, 12, and 24-week post-injury. Poor outcomes were linked to high urinary α1-microglobulin-to-creatinine (α1-MG/C) ratios and active tubular injury scores. Univariate analysis showed a strong link between this ratio and poor renal outcomes, independent of oxalosis severity.

Conclusions

In AKI, CaOx deposition is common despite declining GFR. Factors worsening tubular injury, not just oxalate-to-citrate ratios, are key to understanding impaired renal recovery.

Keywords:

• Acute kidney injury
• acute tubular injury
• hypocitraturia
• calcium oxalate
• tubular calcification

Introduction

The deposition of calcium-containing crystals in the kidney pathology of patients with acute kidney injury (AKI) has been occasionally observed; however, it has not gained adequate attention. Renal oxalosis, also known as oxalate nephropathy (OxN), is a prevalent type of crystalline nephropathy that is typically characterized by kidney injury induced by an abundance of calcium oxalate (CaOx). These crystals damage the kidney tubular epithelial cells, trigger interstitial inflammation and fibrosis, and ultimately result in disease progression with poor kidney outcomes [1–3]. In addition to crystals, the pathogenic effects of urinary soluble oxalate on the kidney have been increasing recognized, especially in patients with chronic kidney disease (CKD). In non-transplanted kidneys, secondary OxN is primarily caused by enterogenic hyperoxaluria resulting from non-genetic conditions, such as fat malabsorption [4,5]. However, the occurrence of CaOx deposition in AKI pathology due to nonenteric risk factors has been sporadically reported and may have been overlooked [6], despite decreased excretion of urinary oxalate due to a decline in glomerular filtration rate.

Hyperoxaluria greatly contributes to the formation of intraluminal CaOx crystals [7]. The excretion of urinary oxalate is primarily regulated by the serum oxalate levels through glomerular filtration, with a small portion secreted by tubular transporters, specifically the solute carrier 26 family 6 (SLC26A6) and eliminated through the gut [8]. However, a cohort study conducted by Mayo Clinic revealed that 23.5% of non-transplanted kidneys with OxN had no identifiable traditional risk factors [6], whereas our case series reported a prevalence of 74.9% [2]. Further, other urinary constituents may contribute to intraluminal crystallization and the internalization of crystals. The level of citraturia, for example, has a significant impact on the formation of either calcium oxalate monohydrate (COM) or calcium oxalate dihydrate crystals. COM formed in the proximal tubule lumen is associated with increased inflammation and cell damage [9]. Nonetheless, tubular crystal casts and plugs rarely obstruct enough nephrons to explain AKI, which prompts the question of whether inflammation is the main factor driving crystal-induced AKI. Previous studies on toxic and post-ischemic AKI have revealed that kidney injuries are largely driven by an unspecific innate immunity response [10–12], rather than merely dependent on the type of crystal involved.

This study aimed to characterize the differences of determinants including urinary parameters, urinary pH, tubular calcification, and tubular SLC26A6 expression between AKI patients with CaOx deposition and serum creatinine-matched AKI controls without oxalosis, and further analyze their effect on oxalosis and potential contribution to adverse long-term kidney outcomes.

Methods

Patients

We enrolled patients who underwent native kidney biopsies with CaOx crystals deposition identified in native kidney biopsies from a cohort of 814 AKI patients who underwent kidney biopsies at the Renal Pathology Center of Peking University First Hospital from January 2011 to December 2020. AKI diagnosis followed the Kidney Disease Improving Global Outcomes criteria [13], and CaOx crystals were confirmed as transparent high-birefringence crystals under polarized light microscopy. Control patients with AKI but without oxalosis were selected, matched for serum creatinine levels. Patients with primary hyperoxaluria as determined by genomic sequencing and family history would not be included. Patients with AKI due to severe glomerulonephritis (GN) were excluded from prognosis analysis (Figure 1). Severe GN was defined as glomerular disease with severe activity, as determined by the ‘glomerular active index’, and was considered the primary contributor to AKI.

Figure 1. Flow chart of the study.

The study was approved by the Ethics Committee of Peking University First Hospital, and written informed consent was obtained from all participants (2021Y042). Figure 1 illustrates the study’s flow chart. The study follows the STROBE guidelines (Supplementary file ‘STROBE checklist’).

Evaluation of Clinical and laboratory parameters

Clinical and laboratory parameters were assessed. Demographic information and medical history, including hypertension, diabetes mellitus, CKD, nephrolithiasis, alcohol, and smoking habits, were extracted from the electronic medical system. Traditional factors associated with hyperoxaluria, such as diarrhea, malabsorption, excessive consumption of oxalate-rich foods, and vitamin C supplements, were recorded. Furthermore, data on other factors that could impact pre-renal glomerular filtration rate were collected. These factors include the use of renin–angiotensin–aldosterone system inhibitors (RAASi), diuretics, and non-steroidal anti-inflammatory drugs (NSAIDs), as well as the presence of hypotension, hypoalbuminemia, blood urea nitrogen (BUN)-to-creatinine ratio (BUN/Cr) >10, and filtration excretion of sodium (FeNa) <1%.

Upon admission, routine biochemical tests were performed, and the daily urine output was meticulously documented. The evaluation of kidney function was based on the measurement of serum creatinine levels and estimated glomerular filtration rate (eGFR) using the CKD-EPI equation [14]. Other parameters including brain natriuretic peptide (BNP), albumin, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) levels were measured. For urine analysis, morning spot-urine parameters were assessed. This included the examination of urine pH, identification of sediments, such as crystals, casts, pyuria, and hematuria. Additionally, the albumin-to-creatinine ratio, glucose levels, N-acetyl-β-D-glucosaminidase (NAG), and α1-microglobulin (α1-MG) were analyzed. To gather comprehensive data on urate and electrolyte levels, 24-h urine collections were conducted. The supernatants of spot-urine samples were utilized to measure the concentration ratios of urinary oxalate and citrate to creatinine (uOx/C, uCi/C) through the implementation of Thermo Fisher IC-MS workflow and an oxalate-citrate assay kit (Colorimetric, ab196990; Abcam, Cambridge, MA). Derivative parameters to further characterize the relationship between urine oxalate, citrate and calcium were calculated as follows: concentration ratio of urinary oxalate to citrate (uOx/uCi), concentration ratio of urinary calcium to citrate (uCa/uCi), uCi/C multiply by uCa/C (uCi/C x uCa/C).

Evaluation of renal pathology

Kidney biopsy specimens underwent a sequential process of staining with hematoxylin and eosin and Masson stain, followed by examination under immunofluorescence, light, and electron microscopy. Polarized light microscopy and electron microscopy were utilized to quantify and localize the CaOx crystals. The severity of crystal deposition was graded based on the number of crystals per section: mild (<5 crystals), moderate (5–10 crystals), or severe (>10 crystals). To detect and measure tubular calcification, the von Kossa staining method was employed, using the von Kossa kit from Jiemei Gene in Shanghai, China. This allowed for the determination of the ‘tubular calcification index’.

To evaluate the extent of tubular injury, tissue inflammation, or fibrotic changes, two indices were created to provide a standardized measurement of the extent of these different renal conditions: an activity index and a chronicity index. These indices, presented in Supplementary Table 1, were developed using a modified Banff working classification as a basis [15]. Briefly, activity index was composed of acute tubular injury score (ATI), acute interstitial injury score (ATI), and acute tubulointerstitial injury score (ATI, calculated as: AT + AI); chronicity index contained chronic tubular injury score (CT), chronic interstitial injury score (CI) and chronic tubulointerstitial score (CTI, calculated as: CT + CI). The glomerular active index was used to evaluate the presence of mesangial injury, endocapillary hypercellularity, segmental sclerosis, and crescents. These findings were then categorized into three groups: no glomerulonephropathy (GN), mild-to-moderate activity, or severe activity. On the other hand, the glomerular chronic index was calculated based on the percentage of globally sclerosed glomeruli. The scores ranged from 0 (sclerosed glomeruli <5), 1 (sclerosed glomeruli 5–10%), 2 (sclerosed glomeruli 11–20%), to 3 (sclerosed glomeruli >20%). This index provided an indication of the degree of glomerular sclerosis present. If a specific GN such as diabetic nodular glomerulosclerosis or focal segmental glomerulosclerosis (FSGS) was identified, the score defaulted to 3 [16]. The pathological diagnosis and semi-quantitative scores were validated by two experienced pathologists.

The expression of tubular SLC26A6 was analyzed using antibodies against SLC26A6 (Santa Cruz, CA) implemented in immunoperoxidase staining on paraffin sections of the kidney biopsy. The findings were quantified as the percentage of positive renal tubules out of the total tubules (%) under a 400× microscope. The expression levels were then categorized as mild (<15%), moderate (15–20%), or severe (>20%).

Treatment, follow-up, and outcomes

Clinical data and follow-up outcomes were retrieved from medical records and conducted via telephone interviews. The correlations of determinants on severity of CaOx deposits and their impact of kidney outcomes were analyzed based on the most recent follow-up within a 6-month period. Patients with their 3-month eGFR <15 mL/min/1.73 m² or kidney failure with replacement therapy (abbreviated as ‘KFRT subgroup’) were recognized as indicating poor long-term outcomes.

Statistical analyses

Normally distributed data are presented as means ± SD, whereas non-normally distributed data are presented as medians (Q25, Q75). Categorical data were summarized as counts and percentages. To compare the clinicopathological and laboratory characteristics of AKI patients with CaOx crystal deposition to the selected controls, the Student’s t-test, Mann–Whitney U test, Kruskal–Wallis test, or Wilcoxon signed-rank test were employed according to appropriateness. The correlation analysis, coupled with univariate regression, was performed using the backward stepwise elimination method to identify risk factors that influenced oxalosis severity and poor kidney outcomes. Regression analyses were reported as odds ratios (ORs) with 95% confidence intervals (CIs). A significance level of p value <0.05 was set. All statistical analyses were conducted using Statistical Software for Social Sciences version 26.0 (SPSS Inc., Chicago, IL).

Results

Clinical features

The prevalence of CaOx crystal deposition was 3.9% of the whole AKI cohort (32 cases). The baseline clinical features of these patients are presented in Table 1 (32 vs. 20 cases). The average age of these patients was 52.8 ± 13.8 years, and there was a higher proportion of males, with a male-to-female ratio of 5.4:1. Most AKI patients with CaOx deposition were linked to tubulointerstitial nephritis caused by nephrotoxic medications, with 63% of these cases being classified as AKI stage 3. The average daily urine output was 1769 ± 628 mL. Nine patients (28%) had traditional predisposing factors for secondary hyperoxaluria. Further, 28 patients (88%) exhibited pre-renal conditions related to renal hypoperfusion (significantly higher than that in the control group, 28 vs. 12 patients, p = 0.04): the use of RAASi in 10 patients (31%), NSAIDs in 6 patients (19%), and diuretics in 14 patients (43.8%). Thirteen patients (41%) reported a history of alcohol abuse, and four (12.5%) had alcohol exposure prior to the onset of the disease. However, all patients denied exposure to toxic alcohol. AKI patients with CaOx deposition tended to have higher rate of comorbidities, including diabetes mellitus (53% vs. 20%, p = 0.02), hypertension (69% vs. 15%, p = < 0.001), alcohol abuse (41% vs. 5%, p = 0.008), and infection (44% vs. 15%, p = 0.04) (Table 1).

Table 1. Baseline features of patients with acute kidney injury (AKI) and CaOx deposition.

Variables	CaOx (n = 32)	Non- CaOx (n = 20)	p Value
Age, year	53 ± 14	52 ± 14	0.75
Men, n (%)	27 (84)	11 (55)	0.03
Prior CKD, n (%)	3 (9)	0	0.15
AKI, n (%)
Stage 1/2/3	6 (19)/6 (19)/20 (63)	2 (10)/5 (25)/13 (65)	0.60
Etiologies of AKI, n (%)
Nephrotoxic drugs	21 (66)	7 (35)	0.05
Idiopathic induced by severe nephrotic syndrome	5 (16)	NA
concurrent with nephrotoxic drugs exposure	2 (6)	NA
Severe proliferative glomerulonephritis (EPGN or CrGN)	9 (28)	NA
Idiopathic tubulointerstitial nephritis	NA	6 (30)
Concurrent with nephrotoxic drugs exposure	NA	5 (25)
Acute tubular necrosis	NA	1 (5)
Comorbidities, n (%)
Diabetes mellitus	17 (53)	4 (20)	0.02
Hypertension	22 (69)	3 (15)	<0.001
Urolithiasis	5 (16)	1 (5)	0.13
Alcohol abuse	13 (41)	1 (5)	0.008
Infection	14 (44)	3 (15)	0.04
Malignancies	2 (6)	1 (5)	1.00
Rheumatologic diseases	3 (9)	0	0.28
Traditional predisposing factors to hyperoxaluria, n (%)	9 (28)	NA
Excessive intake of oxalate-rich food or vitamin C supplement	4 (13)	NA
Refractory diarrhea	5(16)	NA
Gastrectomy	2 (6)	NA
Chronic pancreatitis	1 (3)	NA
Pre-renal conditions to renal hypoperfusion, n (%)	28 (88)	12 (60)	0.04
RAASi	10 (31)	0	0.008
Diuretics	14 (44)	0	<0.001
NSAIDs	6 (19)	3 (15)	1.00
Hypoalbuminemia	10 (31)	1 (5)	0.04
Hypotension	3 (9)	1 (5)	1.00
BUN-to-sCr ratio >10	20 (63)
FeNa < 1%	4/16 (25)	0	0.12
BNP <100	9/24 (38)	7/12 (58)	0.30
Other conditions, n (%)
Antibiotic use within 2 months before biopsy	14 (44)	7 (35)	0.58
Other potential nephrotoxic drugs^a	17 (53)	7 (35)	0.26
Laboratory tests, median (P25, P75)
Biopsy sCr, μmol/L	349.7 (237.5, 587.0)	321.0 (192.8, 466.7)	0.24
Peak sCr, μmol/L	703.3 (465.2, 937.6)	337 (222.0, 618.4)	0.001
BUN-to-sCr ratio, mean ± SD	12.4 ± 5.8
Glycated hemoglobin, %	6.5 ± 1.0	6.1 ± 0.9	0.32
Serum albumin, g/L	36.1 (24.5, 41.2)	39.2 (35.9, 42.6)	0.13
BNP, pg/mL^b	256.5 (42.5, 448.8)	75.3 (25.3, 381.8)	0.44
Urinary pH	6.0 (5.0, 6.5)	6.5 (6.0, 6.9)	0.14
uA/C, g/g.cr	65.5 (15.9, 342.0)	87.8 (28.4, 1216.7)	0.63
uNAG /C, U/g; Increased uNAG, n (%)	28.3 (20.4, 101.8); 20 (74)	28.7 (21.1, 56.6); 13 (77)	0.78; 0.74
Urinary α1-MG /C, mg/g; Increased urinary α1-MG, n (%)	78.9 (28.8, 255.6); 24 (92)	196.5 (24.8, 337.1); 13 (81)	0.56; 1.00
Renal glycosuria, n (%)	17 (53)	11 (55)	1.00
Hematuria, n (%)	22 (69)	6 (30)	0.01
Leukocyturia, n (%)	17 (53)	6 (30)	0.15
Serum IgG, g/L, mean ± SD	12.5 ± 5.5	14.7 ± 4.9	0.22
CRP, mg/L	4.6 (1.2, 17.4)	10.0 (2.2, 17.4)	0.13
ESR, mm/h	45.0 (31.0, 78.5)	68.0 (7.0, 98.5)	0.30

Data are presented as n (percentage), mean ± SD or median (P₂₅, P₇₅); CKD: chronic kidney disease; AKI: acute kidney injury; NA: not available; RAASi: renin–angiotensin–aldosterone system inhibitor; NSAIDs: non-steroidal anti-inflammatory drugs; BUN: blood urea nitrogen; FeNa: fractional excretion of sodium; BNP: B type natriuretic peptide; sCr: serum creatinine; uA/C: urinary albumin-to-creatinine ratio; NAG: N-acetyl-β-D-glucosaminidase (reference: 0.3–12 U/L); α1-MG: α1-microglobulin (reference: <12 mg/L); uNAG/C: urinary NAG to urinary creatinine ratio; α1-MG/C: urinary α1-microglobulin to urinary creatinine ratio; CRP: C-reactive protein; ESR: erythrocyte sedimentation rate

Severe nephrotic syndrome is defined as the presence of edema, proteinuria >6 g/d, and serum albumin <15 g/L. Severe proliferative glomerulonephritis is defined as severe endocapillary proliferative glomerulonephritis and/or crescents observed in >50% glomerulus. Excessive intake of oxalate-rich food or vitamin C supplement is defined as patients receiving >2 g/d vitamin C infusion for >3 d before kidney biopsy or they reported excess oxalate precursor intake. Refractory diarrhea is defined as patient report of stool of Bristol VI or VII type for >3 months. Pre-renal conditions to renal hypoperfusion consists of the use of RAAS inhibitors or diuretics or NSAIDS, or the conditions of hypoalbuminemia or hypotension or BUN-to-sCr ratio > 10. FeNa and BNP are not included in the pre-renal conditions because of missing data.

^a Types of the potential nephrotoxic drugs were summarized in Supplementary Table 6.

^b Only 24 patients had available BNP data.

Upon admission, laboratory tests revealed a mean BUN/Cr ratio of 12.4 ± 5.8. Median serum albumin levels were measured at 36.1 (24.5, 41.2) g/L, and median BNP levels were at 256.5 (42.5, 448.8) pg/mL. Crystalluria of CaOx monohydrate crystals was observed in the urine sediments of 2 patients (6.3%) (Figure 2(A,B)), whereas sterile leukocyturia was present in 17 patients (53%). Renal glycosuria, elevated urinary levels of NAG, and α1-MG were frequently observed. The urine pH was measured at 6.0 (5.0, 6.5). Among AKI patients with CaOx deposition, 72% (23 cases) were attributed to tubulointerstitial nephritis based on clinical evaluation. When comparing to the selected non-oxalosis AKI control induced by tubulointerstitial nephritis, these patients demonstrated no significant difference in the daily urine electrolyte excretion but had the higher level of urinary oxalate (median, 0.2875 mmol/L, p < 0.001), an increased oxalate-to-citrate ratio (0.7110 ± 0.3972, p = 0.005) (Table 2).

Figure 2. CaOx crystal cast and intrarenal depositions. Immunohistochemistry staining showed the expression of SLC26A6 transporter in (A and B) urinary sediments of a cast containing CaOx crystals. (C and D) Intraluminal and intracellular CaOx crystals (original magnification, ×5000). (E) Tubular expression of SLC26A6 transporter in a patient with severe oxalosis and (F) another patient with minimal change disease as the control (original magnification, ×200). (G) CaOx crystals under polarized microscopy (original magnification, ×200). (H) von Kossa staining showed calcium-containing crystals.

Table 2. Comparison of clinical features among AKI patients with oxalosis and non-oxalosis.

	AKI with oxalosis (n = 32)	AKI w/o oxalosis (n = 20)	p Value
AKI Stage 3, n (%)	62.5	65.0	0.86
sCr at biopsy, μmol/L	349.7 (237.5, 587.0)	356.9 ± 206.6	0.24
Urine output, mL	1769 ± 628	1850 (1500, 2400)	0.55
Urinary pH	6.0 (5.0, 6.5)	6.2 ± 0.7	0.39
24-h urine parameters, mmol/24 h
Sodium	109.6 ± 51.7	74.0 (45.0, 170.8)	0.78
Potassium	31.7 ± 15.5	41.1 ± 19.4	0.12
Chloride	100.4 ± 46.7	90.7 (32.0, 173.6)	0.69
Calcium	1.2 ± 0.7	0.7 (0.6, 2.1)	0.96
Phosphorous	12.2 ± 7.8	12.2 (6.5, 20.0)	0.84
Magnesium	2.9 ± 1.6	2.5 (1.8, 5.0)	0.84
Uric acid, mol/24 h	2109.5 ± 1064.0	2899.1 ± 1163.8	0.14
Urea nitrogen	217.3 ± 118.3	241.2 ± 119.5	0.67
Spot-urine parameters, mmol/L
Sodium	63.0 ± 22.1	53.2 ± 22.4	0.21
Potassium	17.0 (12.0, 25.5)	20.0 (13.0, 26.0)	0.37
Chloride	57.2 ± 20.5	55.9 ± 26.0	0.87
Calcium	0.8 ± 0.4	0.8 ± 0.5	0.96
Phosphate	7.2 ± 3.7	6.8 ± 4.2	0.82
Magnesium	1.8 ± 0.7	1.6 ± 0.6	0.46
Uric acid,	1144.7 ± 574.5	1364.0 (1242.7, 1926.4)	0.10
Urea nitrogen	110.3 ± 42.9	131.3 ± 47.2	0.32
Oxalate	0.2875 (0.2225, 0.3825)	0.1925 (0.1850, 0.2413)	<0.001
Citrate	0.4775 (0.3725, 0.7213)	0.6400 (0.3363, 0.8913)	0.29
Urinary Ox/Ci ratio	0.7110 ± 0.3972	0.4057 ± 0.2029	0.005
Urinary Ca/Crea ratio	0.7112 (0. 2059, 1.0104)	0.6661 (0.5244, 0.9157)	0.60
Kidney pathology
Tubular calcification index, n^a	11.00 (5.00, 17.00)	0.00 (0.00, 1.00)	<0.001
SLC26A6 staining score, %^b	0.18 ± 0.05	0.14 ± 0.04	0.009
Glomerular global sclerosis^c	1.50 (0, 3.00)	0 (0, 1.00)	0.01
AT	3.50 (2.25, 5.00)	2.00 (1.00, 5.00)	0.45
AI	1.00 (0.00, 2.00)	0.00 (0.00, 1.00)	0.002
ATI	4.00 (2.00, 5.75)	3.67 ± 3.16	0.010
CT	0.50 (0.00, 1.00)	1.00 ± 0.87	0.048^d
CI	0.00 (0.00, 1.00)	1.00 (0.00, 1.540)	0.14
CTI	0.50 (0.00, 2.25)	2.00 (0.00, 3.00)	0.008

Data are reported as number and percentage, mean ± SD or median (P₂₅, P₇₅). p Value, by Wilcoxon signed rank tests.

sCr: serum creatinine; uOx: urinary oxalate; uOx/C: urinary oxalate (mmol/L) to creatinine (mmol/L) ratio; uCi: urinary citrate; uCi/C: urinary citrate (mmol/L) to creatinine (mmol/L) ratio; uCa urinary calcium concentration; uCa/C, urinary calcium (mmol/L) to creatinine (mmol/L) ratio; uCi/C x uCa/C, uCi/C multiply by uCa/C; uCa/uCi: urinary calcium (mmol/L) to citrate (mmol/L); N/A: not available. AT: acute tubular injury score; AI: acute interstitial injury score; ATI: acute tubulointerstitial injury score (calculated as: AT + AI); CT: chronic tubular injury score; CI: chronic interstitial injury score; CTI: chronic tubulointerstitial score (calculated as: CT + CI)

^aPositive von Kossa-stained crystal amounts were calculated per piece of renal tissues; ^bSLC26A6 transporter expression was shown as the percentage of the number of positive renal tubules to total tubules (%) under 400× microscope. ^cPercent of globally sclerosed glomeruli (score 0, <5%; score 1, 5–10%; score 2, 11–20%; score 3, >20%; if a specific glomerulopathy such as diabetic nodular glomerulosclerosis or focal segmental glomerulosclerosis (FSGS), was identified, the score defaulted to 3). ^d p Value within the range of 0.045–0.050 was expressed up to three significant decimal places.

Pathological characteristics

Renal pathological findings predominantly included degenerative changes in tubular injury, characterized by diffuse interstitial edema, mild interstitial inflammation, and varying degrees of interstitial fibrosis. Half of the patients displayed tubular necrosis, whereas a few had a naked basement membrane. Only one patient presented with tubulitis (Supplementary Table 2). Intraluminal localization of CaOx crystals was evident in all patients, with intracellular localization observed in five patients (16%) (Figure 2(C,D)). In the cohort, severe, moderate, and mild CaOx crystal deposition was present in 40.6%, 18.8%, and 40.6% of the cases, respectively. Calcium phosphate crystals were easily identifiable (Figure 2(H)). There existed a close relationship between the severity of tubular calcification and CaOx deposition (R = 0.550, p = 0.007). When compared to the selected control, AKI patients with CaOx deposition exhibited higher active tubular injury scores (score 3.5 vs. 2.0, p = 0.01), tubular calcification index (11 vs. 0, p < 0.001), and tubular expression of SLC26A6 (0.18 ± 0.05 vs. 0.14 ± 0.04, p = 0.009) (Table 2 and Figure 2). Subgroup analysis of patients with non-to-moderate vs. severe CaOx deposition (Supplementary Table 3) and non-to-mild versus moderate-to-severe CaOx deposition (Supplementary Table 4) consistently revealed similar findings.

Although a considerable proportion of patients exhibited concurrent GN (including diabetic kidney disease, IgA nephropathy, membranous nephropathy, lupus nephritis, and minimal change disease), only 28% of these AKI patients were attributed to severe GN. Notably, all of these patients exhibited mild CaOx.

Factors contributing to severity of oxalosis deposition and renal outcomes

Univariable regression analysis revealed significant associations between moderate-to-severe CaOx deposition and the following factors: peak serum creatinine ($\mu$ mol/L) (OR = 1.002; 95% CI, 1.000–1.004), nadir eGFR (mL/min/1.73 m²) (OR = 0.896; 95% CI, 0.810–0.991), use of diuretics (OR = 7.700; 95% CI, 2.063–28.742), 24-h urine chloride (mol/24h) (OR = 0.773; 95% CI, 0.633–0.994), 24-h uric acid (mol/24h) (OR = 1.006; 95% CI, 1.001–1.012), 24-h urea nitrogen (mol/24 h) (OR = 1.198; 95% CI, 1.025–1.400), uOx/uCi ratio (mmol/mmol, per 0.01 increase) (OR = 1.023; 95% CI, 1.004–1.042), tubular expression of SLC26A6 (OR = 1.401; 95% CI, 1.102–1.780), tubular calcification index (OR = 1.320; 95% CI, 1.105–1.575), and glomerular global sclerosis (OR = 2.026; 95% CI, 1.233–3.330) (Table 3). Furthermore, use of diuretics (OR = 3.889; 95% CI, 1.038–14.566), urinary citrate levels (mol/L) (OR = 0.996; 95% CI, 0.992–1.000), uCi/C ratio (mmol/mmol, per 0.01 increase) (OR = 0.905, 95% CI, 0.822–0.996), uOx/uCi ratio (mmol/mmol, per 0.01 increase) (OR = 1.026; 95% CI, 1.006–1.047), and tubular calcification index (OR = 1.065; 95% CI, 1.002–1.032) were found to be associated with severe CaOx deposition (Table 3).

Table 3. Univariable regression analysis of risk factor to oxalosis.

	Moderate-to-severe oxalosis (<5 [reference] vs.$\ge$5 crystals per high-power field)		Severe oxalosis (≤10 [reference] vs. >10 crystals per high-power field)
	OR (95%CI)	p Value	OR (95% CI)	p Value
Peak sCr ($\mu$ mol/L)	1.002 (1.000, 1.004)	0.04	1.001 (0.999, 1.003)	0.21
nadir eGFR (mL/min/1.73 m^2)	0.896 (0.810, 0.991)	0.03	0.891 (0.788, 1.007)	0.07
sCr at biopsy ($\mu$ mol/L)	1.002 (0.999, 1.004)	0.24	1.002 (0.999, 1.005)	0.13
eGFR at biopsy (mL/min/1.73 m^2)	0.942 (0.887, 1.000)	0.05	0.9678 (0.914, 1.026)	0.28
RAASi	2.585 (0.665, 10.039)	0.17	3.437 (0.834, 14.165)	0.09
Diuretics	7.700 (2.063, 28.742)	0.002	3.889 (1.038, 14.566)	0.04
NSAIDs	0.670 (0.151, 2.974)	0.60	1.371 (0.298, 6.318)	0.69
24-h urine parameters, mmol/24 h
Sodium	0.949 (0.750, 1.201)	0.66	0.994 (0.961, 1.028)	0.73
Potassium	1.085(0.343, 3.426)	0.89	0.926 (0.834, 1.028)	0.15
Chloride	0.773 (0.633, 0.994)	0.01	0.991 (0.958, 1.024)	0.58
Calcium	0.413 (0.017, 10.200)	0.59	1.450 (0.237, 8.889)	0.69
Phosphorous	4.967 (0.484, 50.980)	0.18	1.054 (0.841, 1.321)	0.65
Magnesium	1.978 (0.046, 85.286)	0.72	2.375 (0.553, 10.167)	0.25
Uric acid (per 10^3 increase)	1.006 (1.001, 1.012)	0.03	1.000 (0.998, 1.001)	0.77
Urea nitrogen	1.198 (1.025, 1.400)	0.02	1.012 (0.990, 1.035)	0.29
Spot-urine parameters
Sodium (mol/L)	1.006 (0.985, 1.026)	0.59	0.994 (0.969, 1.021)	0.67
Potassium (mol/L)	1.001 (0.998, 1.004)	0.63	0.999 (0.995, 1.003)	0.57
Chloride (mol/L)	1.005 (0.987, 1.023)	0.60	0.996 (0.973, 1.018)	0.71
Calcium (mol/L)	1.005 (0.981, 1.029)	0.70	0.997 (0.970, 1.025)	0.83
Phosphate (mol/L)	1.001 (0.994, 1.009)	0.73	1.000 (0.992, 1.008)	0.96
Magnesium (mol/L)	1.000 (0.995, 1.004)	0.84	0.999 (0.993, 1.005)	0.70
Uric acid (mol/L)	0.997 (0.993, 1.002)	0.28	0.996 (0.990, 1.002)	0.22
Urea nitrogen (mol/L)	0.997 (0.991, 1.003)	0.32	0.999 (0.993, 1.005)	0.68
uOx (mmol/L)	1.719 (0.243, 12.179)	0.59	0.557 (0.029, 10.764)	0.70
uCi (mol/L)	0.999 (0.998, 1.001)	0.47	0.996 (0.992, 1.000)	0.03
uOx/C (mmol/mmol, per 0.01 increase)	1.047 (0.980, 1.118)	0.17	0.925 (0.808, 1.060)	0.26
uCi/C (mmol/mmol, per 0.01 increase)	0.997 (0.972, 1.002)	0.83	0.905 (0.822, 0.996)	0.04
uCa/C (mmol/mmol, per 0.01 increase)	0.988 (0.975, 1.001)	0.07	0.998 (0.990, 1.006)	0.64
uOx/uCi (mmol/mmol, per 0.01 increase)	1.023 (1.004, 1.042)	0.02	1.026 (1.006, 1.047)	0.01
uCa/uCi (mmol/mmol)	0.998 (0.983, 1.013)	0.76	1.006 (0.992, 1.020)	0.40
Kidney pathology
SLC26A6 %^a (per 1% increase)	1.401 (1.102, 1.780)	0.006	1.181 (0.963,1.447)	0.11
Tubular calcification index^b	1.320 (1.105, 1.575)	0.002	1.065 (1.002, 1.132)	0.04
Glomerular global sclerosis^c	2.026 (1.233, 3.330)	0.005	1.194(0.727, 1.960)	0.48
AT	1.030 (0.725, 1.462)	0.87	1.305 (0.740, 2.301)	0.36
AI	0.850 (0.699, 1.079)	0.18	1.297 (0.870, 1.935)	0.20
ATI	0.934 (0.800, 1.090)	0.38	1.194 (0.911, 1.565)	0.20
CT	1.510 (0.758, 3.007)	0.24	1.006 (0.422, 2.287)	0.99
CI	1.355 (0.869, 2.113)	0.18	0.907 (0.577, 1.479)	0.70
CTI	1.209 (0.915, 1.596)	0.18	0.967 (0.709, 1.319)	0.83

^aSLC26A6 transporter expression was shown as the percentage of the number of positive renal tubules to total tubules (%) under 400 × microscope. ^bPositive von Kossa-stained crystal amounts were calculated per piece of renal tissues. ^cPercent of globally sclerosed glomeruli(0, <5%;1, 5–10%; 2, 11–20%; 3, >20%;If a specific glomerulopathy such as diabetic nodular glomerulosclerosis or focal segmental glomerulosclerosis (FSGS), was identified, the score defaulted to 3).

uOx: urinary oxalate; uOx/C: urinary oxalate to creatinine ratio; uCi; urinary citrate; uCi/C: urinary citrate to creatinine ratio; uCa/C: urinary calcium concentration to creatinine ratio; uOx/uCi; urinary oxalate to citrate ratio; uCa/uCi; urinary calcium to citrate ratio; SLC26A6%: the percentage of SLC26A6 expression

All AKI patients with CaOx deposition received supportive therapy, and three of them were given potassium citrate supplements. However, renal recovery was delayed, as 43.8%, 31.2%, and 18.8% of AKI patients with CaOx deposition had an eGFR < 15 mL/min/1.73 m² at the 4, 12, and 24-week follow-up post-injury, respectively. Serum creatinine was much higher in CaOx patients at 4-week (243.41[156.00–425.65] vs. 153.00 [122.00–193.88] $\mu$ mol/L, p = 0.02), while the gap was narrowed down at 12-week (161.00 [112.00–271.50] vs. 120.50 [105.75–145.50] $\mu$ mol/L, p = 0.09) and 24-week (134.00 [93.06–183.530] vs. 121.41 [106.00–161.68] $\mu$ mol/L, p = 0.77) (Supplementary Figure 1). When comparing long-term outcomes, it was found that patients in the KFRT subgroup had a high prevalence of severe GN, reaching 80% (Supplementary Table 5), no difference of CaOx deposition severity found within subgroups having various renal outcomes. In the 23 cases of patients with CaOx deposition but without severe GN, higher levels of tubular injury, as indicated by the urinary α1-microglobulin-to-creatinine (α1-MG/C) ratio and the pathologic active tubular injury score, were significantly associated with poor renal outcomes (eGFR < 15 mL/min/1.73 m² at the 3-month follow-up) compared to others (Table 4). Instead of severity of CaOx deposition, univariate regression analysis confirmed a strong association between urinary α1-MG/C ratio and an unfavorable kidney outcome (OR = 1.007) (Table 5).

Table 4. Comparison between AKI patients having CaOx deposition (having no severe glomerulonephropathy) with a 3-month eGFR <15 and ≥ 15 mL/min/1.73 m².

	eGFR (mL/min/1.73 m^2) at 3-month
	<15 (or KFRT) (n = 5)	$\ge$15 (n = 17)	p
Nadir eGFR (mL/min/1.73 m^2), median (P25, P75)	6.2 (5.0, 9.0)	6.2(3.5, 10.4)	0.70
AKI stage 3 (%)	5 (100)	7 (41)	0.14
Diabetes (%)	2 (40)	11 (65)	0.61
Hypertension (%)	4 (80)	12 (71)	1.00
Urolithiasis (%)	2 (40)	5 (29)	1.00
CaOx deposition ≥ 5/HPF (%)	3 (60)	12 (71)	1.00
CaOx deposition >10/HPF(%)	1 (20)	9 (53)	0.32
Tubular calcification index	17 ± 11	11 (4.50, 35.00)	0.54
SLC26A6 % (mean ± SD)	17 ± 8	18 ± 4	0.60
uOx,mmol/L (mean ± SD)	0.40 ± 0.20	0.27 ± 0.06	0.22
uOx/C^a, median (P25, P75)	0.04 (0.01, 0.46)	0.27 (0.22, 0.33)	0.80
uCi mmol/L, median (P25, P75)	0.50 (0.32, 1.71)	0.45 (0.34, 0.62)	0.62
uCi /C^b, median (P25, P75)	0.25 (0.01, 0.40)	0.06 (0.03,0.20)	0.68
uCa/C^c, median (P25, P75)	0.35 (0.12, 1.75)	0.90 (0.29,1.03)	0.43
uOx/uCi^d, median (P25, P75)	0.88 ± 0.68	0.61 ± 0.29	0.44
uCa/uCi^e, median (P25, P75)	3.28 (0.37, 149.60)	8.32 (2.57, 42.36)	0.56
uA/C, mg/g	1719 (963, 1766)	27 (14, 105)	0.03
uA/C to uα1-MG	2.15 (1.42, 5.94)	0.48 (0.18, 2.27)	0.19
uNAG/C, U/g, median (P25, P75)	68 (43, 85)	27 (15, 57)	0.31
uα1-MG /C, mg/g, median (P25,P75)	297 (237, 571)	66 (44, 104)	0.009
Pathological scores
Glomerular active index			0.14
No GN	1 (20)	11 (65)
Mild-to-moderate activity	4 (80)	6 (35)
Glomerular sclerosis^f	3.0 (1.0, 3.0)	1.0 (0, 3.0)	0.22
AT	4.0 (3.0, 4.0)	1.0 (1.0, 3.0)	0.01
AI	2.0 (1.0, 2.5)	1.0 (0, 2.0)	0.34
ATI^f	6.0 (4.0, 7.0)	4.0 (1.0, 4.0)	0.12
CT	1.0 (0, 2.0)	0.0 (0, 1.0)	0.52
CI	2.0 (0, 4.5)	1.0 (0, 2.0)	0.32
CTI^g	3.0 (0, 6.5)	2.0 (0, 3.0)	0.36

Data are presented as n (percentage), mean ± SD, or median (P₂₅, P₇₅).

^aExpressed as the ratio of urinary oxalate (mmol/L) to urinary creatinine (mmol/L); ^bexpressed as the ratio of urinary citrate (mmol/L) to urinary creatinine (mmol/L); ^cexpressed as the ratio of urinary calcium (mmol/L) to urinary creatinine (mmol/L); ^dexpressed as the ratio of urinary oxalate (mmol/L) to urinary citrate (mmol/L); ^eexpressed as the ratio of urinary calcium (mmol/L) to urinary citrate (mmol/L); ^fATI = AT + AI; ^gCTI = CT + CI.

KFRT: kidney failure requiring replacement therapy; GN: glomerulonephritis; uOx: urinary oxalate; uCi: urinary citrate; uCa: urinary calcium; uNAG/C: urinary N-acetyl-β-D-glucosaminidase to urinary creatinine ratio; α1-MG/C: urinary α1-microglobulin to urinary creatinine ratio; AT: acute tubular injury score; AI: acute interstitial injury score; CT: chronic tubular injury score; CI: chronic interstitial injury score

Table 5. Univariate regression analysis of poor long-term kidney outcome (eGFR < 15 mL/min/1.73 m² or on KFRT vs. eGFR ≥ 15 mL/min/1.73 m² [reference] at 3 months) in patients having no glomerulonephritis.

	OR (95% CI)	p
Nadir eGFR (mL/min/1.73 m^2)	0.985 (0.751, 1.291)	0.91
Diabetes	0.364 (0.047, 2.817)	0.33
Hypertension	1.667 (0.147, 18.874)	0.68
Urolithiasis	1.600 (0.202, 12.694)	0.66
History of medications
RAASi	7.333 (0.661, 81.365)	0.11
Diuretics	1.600 (0.202, 12.694)	0.66
NSAIDs	7.000 (0.791, 61.977)	0.08
CaOx deposition ≥ 5/HPF	0.625 (0.079, 4.959)	0.66
CaOx deposition >10/HPF	0.222 (0.020, 2.424)	0.22
SLC26A6 % (per 1% increase)	0.922 (0.695, 1.223)	0.57
Tubular calcification index	1.006 (0.939, 1.079)	0.86
uOx ($\mu$ mol/L)	1.011 (0.998, 1.025)	0.11
uOx/C^a (per 0.01 increase)	1.048 (0.974, 1.127)	0.21
uCi ($\mu$ mol/L)	1.001 (0.999, 1.003)	0.26
uCi /C^b (per 0.01 increase)	1.001 (0.974,1.028)	0.97
uCa/C^c (per 0.01 increase	0.998 (0.986, 1.010)	0.75
uOx/uCi^d (per 0.01 increase)	1.016 (0.991, 1.041)	0.22
uCa/uCi^e	1.009 (0.993, 1.025)	0.30
uA/C, g/g	1.086 (0.695, 1.697)	0.72
uNAG/C (U/g)	1.007 (0.986, 1.028)	0.53
uα1-MG /C (mg/g)	1.007 (1.000, 1.013)	0.04
Pathological scores
Glomerular active index
No GN	reference
Mild-to-moderate activity	7.333 (0.661, 81.365)	0.11
Glomerular global sclerosis^f	1.731 (0.718, 4.173)	0.22
AT	3.524 (1.012, 12.272)	0.48
AI	1.086 (0.631, 1.869)	0.77
ATI^f	1.303 (0.890, 1.908)	0.17
CT	1.758 (0.553, 5.590)	0.34
CI	1.669 (0.820, 3.397)	0.16
CTI^g	1.344 (0.858, 2.105)	0.20

^a The ratio of urinary oxalate (mmol/L) to urinary creatinine (mmol/L); ^bthe ratio of urinary citrate (mmol/L) to urinary creatinine (mmol/L); ^cthe ratio of urinary calcium (mmol/L) to urinary creatinine (mmol/L); ^dthe ratio of urinary oxalate (mmol/L) to urinary citrate (mmol/L); ^ethe ratio of urinary calcium (mmol/L) to urinary citrate (mmol/L); ^fATI = AT + AI; ^gCTI = CT + CI.

KFRT: kidney failure with replacement therapy; RAASi: renin–angiotensin–aldosterone system; NSAIDs: non-steroidal anti-inflammatory drugs; GN: glomerulonephritis; uOx: urinary oxalate; uCi: urinary citrate; uCa: urinary calcium; uNAG/C: urinary NAG to urinary creatinine ratio; α1-MG/C: urinary α1-microglobulin to urinary creatinine ratio; AT: acute tubular injury score; AI: acute interstitial injury score; CT: chronic tubular injury score; CI: chronic interstitial injury score

Discussion

Our study offers crucial clinical and pathological insights into patients with AKI who exhibit intrarenal CaOx deposition, a condition observed in 3.9% of our AKI patient cohort. These patients characterized by significant CaOx deposition, not only have compromised renal function but also present with unique features: an elevated oxalate-to-citrate ratio associated with hypocitraturia and increased tubular expression of the SLC26A6 transporter; a higher tubular calcification index; and a status of intrarenal ischemia indicated by reduced urinary excretion of chloride and citrate, along with a high prevalence of glomerular global sclerosis and frequent use of diuretics. However, the relatively low incidence of crystalluria in these patients indicates that the clinical diagnosis of CaOx deposition is often underappreciated. Oxalosis-enabling conditions in AKI patients suggest the involvement of underlying chronic kidney injury related to comorbidities (diabetes, hypertension, etc.) or renal ischemic lesions (glomerulosclerosis), which might explain the delayed remission in these patients. Rather than severity of oxalosis, our findings suggest that patients with more pronounced tubular injury experience worse kidney outcomes. We propose that tubular injury is a consequence of a multifactorial response to CaOx crystals, exacerbated by intrarenal ischemia, which impairs the renal tubules’ capacity to prevent crystallization. These observations merit further investigation [2,7].

Hyperoxaluria has long been recognized as a critical factor in the development of secondary oxalosis. Unlike in CKD, where SLC26A6-mediated enhancement of enteric oxalate secretion occurs, AKI patients are unable to effectively compensate for oxalate regulation in the intestines, resulting in an increased oxalate burden [17]. The upregulation of tubular SLC26A6 expression in response to a significant decrease in GFR can result in an increased presence of oxalate in the tubular fluid, specifically reaching the distal tubules where calcium crystallization primarily takes place. However, our study has unveiled an interesting finding that patients with intrarenal CaOx deposition exhibited lower urinary oxalate excretion compared to those without crystal. It follows that the presence of inhibitors such as citrate and uromodulin that prevent crystal formation in the urine, plays an essential role in the genesis of oxalosis. As a result, massive CaOx crystals can form. The higher ratio of oxalate-to-citrate observed in this study indicated the contribution of urinary citrate to inhibition of CaOx deposition. Citrate plays an essential role in preventing the formation of CaOx crystal by binding to calcium and oxalate, forming soluble complexes that are excreted in the urine. Citrate is synthesized in the mitochondria and is crucial for energy production [18]. It is produced by the liver, filtered by the kidneys, and affected by factors, such as bone metabolism, kidney clearance, metabolic acidosis, and hypokalemia [19–20]. Medications and certain conditions can further decrease urinary citrate levels. Avid reabsorption of citrate in the tubules may indicate an energy imbalance in AKI patients [18]. Restoring plasma citrate by reducing its excretion in urine may have positive effects on energy utilization. In patients with gastrointestinal malabsorption, the lack of daily urinary citrate excretion contributes to the development of oxalosis, along with secondary hyperoxaluria [21,22]. The use of angiotensin-converting enzyme inhibitors can cause hypocitraturia, unrelated to acidosis or hypokalemia [23]. Studies have shown that enalapril reduces urinary citrate excretion and increases renal cortical ATP citrate lyase activity. The activation of the proximal tubular Na/H antiporter by angiotensin II and subsequent changes in intracellular pH may affect urinary citrate excretion and renal citrate metabolism [24]. Despite the increased activity of the succinate transporter NaDC-1, urinary citrate levels remain low [25]. Moreover, citrate attachment to crystal surfaces enhances the protective effect of other inhibitors, including the Tamm–Horsfall protein [26].

From the study result, the calcium-containing crystal was more evident than the CaOx crystal, and the tubular calcification index on pathology was significantly elevated in AKI patients with CaOx deposition. We believe that hypocitraturia weakens the defenses against the formation of both COM crystal and calcium phosphate crystals, leading to increased cytotoxicity and inflammation [27,28]. Administration of potassium citrate increases urinary citrate levels, and alleviates renal pathological changes possibly through the expression of NaDC-1, ultimately reducing the occurrence of nephrolithiasis [29]. Furthermore, the activation of the SLC26A6 transporter not only functions as a 1Cl⁻/1oxalate transporter but also contributes to calcium phosphate precipitation by acting as an ion exchanger of 1Cl⁻/2HCO3⁻ [30]. This leads to increased HCO3⁻ excretion and urine alkalization, which in turn predisposes to phosphate crystallization. The buildup of phosphate and calcium in the tubular fluid can be attributed to decreased urine flow caused by transient dehydration during glomerular hypoperfusion. This condition is commonly induced by the combined use of diuretics, RAASi, and NSAIDs, particularly in the presence of severe hypoalbuminemia. Moreover, the increased reabsorption of salt and water in both the proximal tubule and descending limb of Henle’s loop further reduces phosphorus reabsorption due to the limited permeability of calcium and phosphate in this loop segment. The subsequent formation of calcium phosphate crystals can directly damage tubular epithelial cells. Oxidative stress from reduced extracellular fluid exacerbates CaOx crystal formation in hyperoxaluric rat kidneys [31]. Damaged tubular cells linked to pre-renal glomerular ischemia and expressing surface-bound hyaluronan and osteopontin, bind crystals, leading to adhesion and activation of NLRP3 inflammasomes, thereby worsening AKI [12].

Our study had some limitations. The sample size of our study was too small to better stratify the severity of CaOx deposition and to conduct multivariate regression to further analyze the relationships of important influencing factors (including crystal severity, higher urinary α1-MG/C ratio, and active tubular injury scores) and kidney outcomes in patients with AKI. We were unable to examine plasma oxalate and citrate owing to the retrospective nature of this study. Additionally, based on the nature of AKI that inhibitors against crystals formation are dramatically reduced, the influences of urinary level and function of uromodulin should be considered. Lastly, we used creatinine as the only matching criterion between patients with CaOx deposition and the control group since the sample size was small, which might lead to selection bias due to other confounding factors like underlying causes of AKI, age, and comorbidities. A more comprehensive balancing of the potential confounding factors would be achieved in further cohort studies with larger sample size.

In conclusion, CaOx deposition is relatively frequent even as the glomerular filtration rate decreases in AKI. This delays and impairs renal function restoration. Beyond the increased urinary oxalate-to-citrate ratio, instead of CaOx deposition, it is crucial to consider factors that exacerbate active tubular injury, as these hinder the recovery of renal function. Understanding the mechanisms can help clinicians develop proactive interventions to minimize negative effects of CaOx formation in this population.

Authors’ contributions

WY, TZ, XC, SW, YW, and TS participated in data collection and clinical follow-up. WY, TZ, and TS made the draft, and TZ and TS revised the draft. SW made the pathological diagnosis. TS designed and revised the manuscript. All authors read and approved the final manuscript.

Acknowledgements

The authors thank the Editage (https://www.editage.com/) for the help of English editing.

Disclosure statement

No potential conflict of interest was reported by the authors.

Data availability statement

All data relevant to the study are included in the article or uploaded as supplementary information.

Additional information

Funding

This research was supported by grants from the National Science and Technology Major Projects for major new drugs innovation and development (2017ZX09304028), National High Level Hospital Clinical Research Funding (Interdepartmental Clinical Research Project of Peking University First Hospital; grant number: 2022CR09) and CAMS Innovation Fund for Medical Sciences (2019-I2M-5-046).