Risk factors for acute kidney injury (AKI) in infants with melamine-associated urolithiasis and follow-up: a multi-center retrospective analysis

Abstract

An epidemic of urinary tract stones was noted among infants in China, 2008. This event was believed to be associated with consumption melamine-contaminated powdered formula. The patients with symptoms and clinical manifests had already been analyzed in our previous studies. In this study, our aim is to investigate the risk factors of melamine-associated acute kidney injury (AKI) and the potential relationship toward children growth in our five years follow-up. A total of 619 infants with melamine-associated urolithiasis were admitted into 20 different hospitals in the Gansu province, China. All clinical data were divided into AKI and control groups according to the occurrence of AKI. Univariate and multivariate analyses were performed with a logistic regression model to assess the independent risk factors of AKI. Logistic regression analysis revealed that the odds ratio (OR) of AKI was 19.62 in the group of infants who consumed Sanlu® milk powdered infant milk formula. A higher prevalence of AKI was observed in infants age of 6–11 months (OR: 9.59, p < 0.01) and 12–17 months (OR: 5.06, p < 0.01). Multivariate analysis also indicated that any one symptoms of upper respiratory tract infection (URTI), diarrhea, dehydration and fever (OR: 4.29, p < 0.01) were independent risk factors of AKI. Therefore, this study demonstrated that high melamine infant formula (Sanlu® milk powdered infant formula), age (6–17 months) and symptoms of URTI, diarrhea, dehydration or fever were risk factors of AKI in infants with melamine-associated urolithiasis.

• Acute kidney injury
• infants
• melamine
• risk factors
• urolithiasis

Introduction

An epidemic of urinary stones affecting infants after consumption of melamine-tainted milk powder occurred in 2008 in China. Since most of the stones were soft and can be expelled spontaneously by conservative treatment, the affected infants were mostly asymptomatic and had no sequela after hospital discharge.1–4 However, presence of urinary stones in bilateral ureteropelvic junctions, bladder neck and urethra could induce acute kidney injury (AKI) with oliguria, anuria, anasarca, raised serum creatine levels and eventually lead to severe renal damages if effective management is not provided promptly.1,3 Moreover, recent studies appeared that children with melamine-related urinary stones were prone to be lower heights and weights than normal.5 We have thus presented the data of AKI infants caused by melamine-contaminated milk formula powder in this study, with discussion of risk factors for AKI, which also might be the potential causes for growth insufficient in children.

Materials and methods

Participants

From April 2008 to November 2008, we performed a retrospective analysis of the clinical data of 619 infants with melamine-associated urolithiasis in the Gansu province by identifying all infants who had a history of consuming melamine-contaminated infant formula, of them 47 children were diagnosed with melamine-induced AKI and treated in seven institutions in Gansu Province, China, including Lanzhou University Second Hospital, First Hospital of Chinese People's Liberation Army, People's Hospital of Gansu Province, Tenth Hospital of Chinese People's Liberation Army, Wuwei City Liangzhou District People's Hospital, Jiuquan City People's Hospital and The General Hospital of Coal Industry Limited Corporation of Jingyuan. Urinary tract calculi of infants were confirmed by ultrasonography. Data on demographics, clinical features, outcomes of ultrasound and radiographic investigations, treatment details and outcomes were collected from hospital medical records. Melamine-induced urinary stones were soft and easy to remove. So conservative treatment was the first-line therapy, which included intravenous fluid infusion and use of diuretics to increase urine output (30 mL/kg of 0.9% saline solution and 1 mg/kg furosemide), urine alkalization (to pH > 6.5) with 5% sodium bicarbonate (5 mL/kg daily diluted in 1.4 of 5% glucose solution) and urethral catheterization if necessary. Antibiotics were administered to patients with fever, diarrhea or urinary tract infection. Three infant patients with definite anatomic malformations of the urinary system (one with renal malrotation and two with congenital ureteropelvic junction obstruction) were excluded.

AKI was diagnosed and defined according to the new diagnostic criteria in our clinic procedures6: a time constraint of 48 hours based on the variation of serum creatinine with modern analyzers increments of 0.3 mg/dL (25 μmol/L) or by 50% to normal; and (or) urine volume in six hours increase less than 0.5 mL/(kg h).

For patients presenting the AKI manifest above would be considered, surgical intervention to relieve the obstruction was performed immediately. Each patient was evaluated for apparent obstruction and its location. To those with obstructing bladder or urethral calculi, cystoscopic or urethroscopic lithotripsy was preformed. Retrograde catheterization into the ureter via cystoscope was performed if AKI was caused by bilateral ureteral and/or renal calculi obstruction. Ureterolithotomy or percutaneous nephrostomy was performed in hospitals lacking minimally invasive endoscopic expertise.

This series of studies were approved by the Ethics Committee of Second Hospital of Lanzhou University. Written consents for the study were obtained from all parents or guardians.

Group design

The infant patients were divided into two groups (AKI group and the control group) according to the presence of obstructive AKI. Possible risk factors of the AKI caused by melamine-contaminated formula were attempted to investigate. The exploratory factors of the analysis included gender, age, brand of milk powder, powder intake time (PIT); symptoms of upper respiratory tract infection (URTI), diarrhea, dehydration and fever history (up to one week before hospitalization). Melamine-contaminated infant formula was classified into two types set by the Chinese Administration of Quality Supervision, Inspection and Quarantine7:

1. High tainted milk powder: milk samples' melamine was detected above 1000 mg/kg (e.g. SanLu® milk powder: 2563 mg/kg).

2. Low tainted milk powder: milk samples' melamine was detected under 1000 mg/kg (other melamine-contaminated infant formula).

Follow-up design

We have already followed up the children suffering from melamine-induced AKI (38 children), twice (18 and 36 months) and the 5 year follow-up were performed during May to July 2013. All children were measured their BMI and Kaup index. Ultrasound examination of urinary tract was performed using an ultrasonography system (ProSound SSD-5000SV, Aloka, Tokyo, Japan) with an attached scanner monitor (5–6 MHz). BUN and Cr in blood samples were analyzed using a 7600-010 Automatic Analyzer (Hitachi, High-Tech Science Systems Corporation, Tokyo, Japan).

Statistical analysis

Data were analyzed using the SPSS 17.0 software package (SPSS Inc., Chicago, IL). Numerical variables that were normally distributed were expressed as mean ± SD and analyzed by independent sample t test. Chi-square tests or continuity corrected Chi-square tests were used in count data. Pearson correlation analysis was used to evaluate the correlation of the data. A logistic regression was employed to relate the risk factors to the occurrence of the event. We took the occurrence of AKI as the dependent variable and all other factors as independent variable. Single factor analysis was performed to obtain the odds ratio (OR), 95% confidence interval and p value. The factors that presented statistical significance were considered to be the risk factors to AKI occurrence and would be included in the logistic regression model. After the risk factors were obtained and ensured, multivariate logistic regression analysis was performed to assess the independence of risk factors for AKI occurrence accordingly. All statistical tests of hypotheses are two-sided, and p value less than 0.05 were considered statistically significant.

Results

General data

In this study, a total of 619 infant patients with urolithiasis were enrolled. Forty-seven patients with AKI (34 male and 13 female) were assigned into AKI group and 572 patients into control group. The mean age of AKI group was 10.83 ± 5.11 months. PIT of melamine-contaminated infant formula was 9.30 ± 3.89 months (ranged from 3 to 23 months). All infant patients complained of dysuria, oliguria or anuria; 23 experienced diarrhea; 18 had fever, 16 had dehydration and 15 suffered infection in upper respiratory tract (URTI). Gender, age, PIT, URTI, fever, diarrhea, dehydration and milk powder brand (Sanlu® milk powder) were included as potential risk factors of AKI according the data of two groups. Some bivariate factors were suitable to develop into exposed (yes) and unexposed subgroup (no). Single factor logistic regression analysis was performed dependently on these factors.

The results of Table 1 showed that the infants who had consumed Sanlu® milk powdered formula had 19.62 times (95% CI: 2.68–143.39) higher incidence for AKI than those fed with other formula. Compared to unexposed group, the OR values of URTI, fever, diarrhea and dehydration in exposed groups were 53.86 (95% CI: 23.08–125.68), 294.71 (95% CI: 37.85–2294.69), 19.10 (95% CI: 9.00–40.54), 10.26 (95% CI: 4.93–21.34), respectively (Table 1).

Table 1. General data.

Risk factor		AKI (%) (n = 47)	Control (%) (n = 572)	Odds ratio (95% CI)		p Values
Gender	M	34 (72.3)	374 (65.4)	1.39 (0.71–2.68)	0.97	0.33
	F	13 (27.7)	198 (34.6)	1.00
Age (month)	–	10.83 ± 5.11	16.77 ± 11.37	–	–	0.000
PIT (month)	–	9.30 ± 3.89	11.31 ± 8.05	–	–	0.008
SanLu® powder	Y	46 (97.9)	401 (70.1)	19.62 (2.68–143.39)	24.08	0.000^a
	N	1 (2.1)	171 (29.9)	1.00
Diarrhea	Y	23 (48.9)	10 (1.7)	53.86 (23.08–125.68)	91.80	0.000^a
	N	24 (51.1)	562 (98.3)	1.00
Dehydration	Y	16 (34.0)	1 (0.2)	294.71 (37.85–2294.69)	80.78	0.000^a
	N	31 (66.0)	571 (99.8)	1.00
Fever	Y	18 (38.3)	18 (3.1)	19.10 (9.00–40.54)	32.23	0.000^a
	N	29 (61.7)	554 (96.9)	1.00
URTI	Y	15 (31.9)	25 (4.4)	10.26 (4.93–21.34)	52.17	0.000^a
	N	32 (68.1)	547 (95.6)	1.00

Note: ^aContinuity corrected Chi-square tests.

Single factor analysis

Age, PIT and AKI occurrence

The age group was divided into five dummy variables (<6 months, 6 ∼ 11months, 12 ∼ 17 months, 18 ∼ 23 months and 24 months ∼) and PIT factor was divided into four dummy variables (<3 months, 3 ∼ 5 months, 6 ∼ 11 months and 12 months ∼).

After adjusting for age, logistic regression was performed. The results of age subgroup showed that compared to the morbidity of 2.1% in subgroup of 24 months ∼, 6 ∼ 11 months subgroup presented a prevalence of 57.4% (OR: 9.22, 95% CI: 3.17–26.84, p < 0.01) in susceptible to AKI. Another subgroup of 12 ∼ 17 months 34.0% (OR: 5.70, CI: 1.87–17.35, p < 0.01) also presented similar results. While no difference was found in the <6 months (2.1%, OR: 3.76, 95% CI: 0.23, 61.50) and 18 ∼ 23 months (4.3%, OR: 2.98, 95% CI: 0.41–21.65) subgroups.

For the PIT, compared to <3 months subgroup, 6 ∼ 11 months subgroup presented a AKI prevalence of 61.7% (OR: 2.075, 95% CI: 1.127–3.822); the 12 months ∼, 3 ∼5 months subgroup were 25.5% (OR: 0.968, 95% CI: 0.353–2.650) and 8.5% (OR: 0.958, 95% CI: 0.167–5.489), respectively (Table 2).

Table 2. Regression analysis of age, powder-intake time and AKI occurrence in infants.

					95% CI
		AKI group (%)	Control group (%)	Odds ratio	95% lower	95% upper	p Value
Age (months)  = 25.82, p < 0.01	<6 m	1 (2.1)	37 (6.5)	3.76	0.23	61.50	0.35
	6 ∼ 11 m	27 (57.4)	172 (30.1)	9.22	3.17	26.84	0.000
	12 ∼ 17 m	16 (34.0)	165 (28.8)	5.70	1.87	17.35	0.002
	18 ∼ 23 m	2 (4.3)	59 (10.3)	2.98	0.41	21.65	0.28
	24 m ∼	1 (2.1)	139 (24.3)	1	–	–
PIT(month)  = 5.66, p = 0.017	12 m ∼	12 (25.5)	217 (37.9)	0.968	0.353	2.650	0.968
	6 ∼ 11m	29 (61.7)	250 (43.7)	2.075	1.127	3.822	0.019
	3 ∼ 5m	4 (8.5)	71 (12.4)	0.958	0.167	5.489	0.958
	<3 m	2 (4.3)	34 (5.9)	1	–	–	–

Symptoms and AKI occurrence

To our knowledge, there is an intimate connection amongst symptoms, i.e: fever could be caused by URTI and diarrhea and dehydration usually could be caused by diarrhea. We used a multivariate correlation matrix to assess correlation amongst four symptoms. According to the correlation matrix, associations between four symptoms were various, with correlation coefficients ranging from 0.259 to 0.708. High correlation between diarrhea and dehydration existed (r = 0.708). Moderate correlations existed between fever and URTI (r: 0.40–0.70), also fever between and diarrhea. Low correlation existed between the remainders(r < 0.4) (Table 3).

Table 3. Correlation matrix of presenting symptoms.

	Fever	Diarrhea	Dehydration	URTI
Fever	1.000	0.433	0.338	0.524
Diarrhea	–	1.000	0.708	0.259
Dehydration	–	–	1.000	0.277
URTI	–	–	–	1.000

Note: All p value < 0.01.

To avoid the possible multicollinearity-bias in the regression model, the frequency of symptoms was cited accordingly. Among all infants with clinical feature, there were 17 cases of diarrhea combined with dehydration, and 21 cases were URTI with fever, 3 symptoms occurred in 8 cases and 4 symptoms in 6 cases simultaneously. The dose–reaction relationship was indicated significantly between the symptoms frequency and AKI occurrence. In particular, symptoms frequency (3) showed the highest OR (OR: 249.200, 95% CI: 28.819–2154.814) of all other frequencies (Table 4).

Table 4. Correlation between symptom frequency and AKI.

				95% CI
Frequency	AKI (%)	Control (%)	Odds ratio	95% Lower	95% Upper
4	5 (10.6)	1 (0.2)	178.000	19.576	1618.541
3	7 (14.9)	1 (0.2)	249.200	28.819	2154.814
2	11 (23.4)	11 (1.9)	35.600	13.352	94.919
1	9 (19.1)	25 (4.4)	12.816	5.115	32.113
0	15 (31.9)	534 (93.4)	1	–	–

Note:  = 113.85, p = 0.000.

Multivariate analysis

To assess the independence of risk factors, also the correlation intensity and tendency toward AKI occurrence, a multivariate regression model was necessary and performed. All original significative factors including age (6 ∼ 11 and 12 ∼ 17 months), PIT (6–11 months), high melamine concentration milk powder (Sanlu® milk powder) and symptoms frequency were tend to select into our regression model. However, considering the infants sample were the same between the age of: 6 ∼ 11 or 12 ∼ 17months group and PIT: 6 ∼ 11 months group; we have combined these groups into two age groups: (6 ∼ 11 months) and (12 ∼ 17months) group to avoid bias. Since the OR of symptoms frequency (2, 3, 4) varied in a large range and might cause the unstable and error in the model. Symptoms frequency (1) was appropriate and reasonable. At last, the 6 ∼ 11 months, 12 ∼ 17months group, symptoms frequency (1) and high melamine concentration milk powder (Sanlu® milk powder) were accommodated in our regression model.

The multivariate logistic regression analysis was done with a forward stepwise method. All four factors entered the logistic regression as independent predictors, since the OR values did not vary a lot, symptoms frequency (1), Sanlu® milk powder, 6 ∼ 11 months and 12 ∼ 17 months were all found to be independent risk factors for AKI occurrences of infants with melamine-associated urolithiasis (Table 5).

Table 5. Logistic regression analysis on AKI caused by melamine-associated urolithiasis.

						95% CI
	B	Standard error	Wald	p Value	Odds ratio	95% Lower	95% Upper
Constant	–6.848	1.123
Symptoms Frequency (1)	1.455	0.457	10.155	.001	4.29	1.75	10.49
Sanlu® milk powder	2.982	1.020	8.550	0.003	19.73	2.67	145.66
6 ∼ 11 m	2.260	0.553	16.727	0.000	9.59	3.25	28.33
12 ∼ 17 m	1.620	0.575	7.952	0.005	5.06	1.64	15.59

Follow-up result

So far, a total of 38 (81%) AKI patients (60.50 ± 5.27 months) were successfully followed up for three times (18, 36, 60 months, respectively). All had normal BUN and Cr during three times follow-up. Four of five patients who had residual renal calculi on 18 and 36 months follow-up proved to be calculi-free now, one of the patients remain had a small residual renal stone. We calculated the heights and weights used the Kaup index (kg/cm × 2/104) for 38 AKI children, the mean Kaup index was (15.12 ± 1.26), which at the low edge of the normal index (15–18). Whether the issue has correlation to AKI risk factors or melamine still needs further study.

Discussion

AKI occurs at early phase of acute renal failure. The diagnosis of AKI is based on decreased glomerular filtration rate, increased serum creatinine level or cystatin C level, and oliguria. Early diagnosis and treatment of AKI reduces mortality and morbidity of acute renal failure. Therefore, early diagnosis and management AKI are essential. It has been reported that the total morbidity of AKI was 8 ppm, although data on morbidity in infants remains unclear.8,9 In our study, a morbidity of AKI infants with melamine-associated urolithiasis was 7.6%, and there was no report about this to our knowledge. Most of the urolithiasis of melamine were be expelled via conservative treatment. However, a few calculi were located in the pelvis and ureters. Obstruction of urinary tract was therefore possibly leading to ipsilateral hydronephrosis and AKI.

From our previous retrospective analysis of 22 infants,3 our findings indicated the main cause of this event was to intake the high melamine concentration milk powder (Sanlu® milk powder). Symptoms of fever, diarrhea, URTI and dehydration were found to be remote causes. Base on this multicenter retrospective study, we found that milk powder rich in melamine was not only the main cause of AKI but it remained as a high risk factor to urolithiasis.

We also observed that some infants (6 ∼ 11 months and 12 ∼ 17 months) had a higher prevalence to AKI. This could be explained by the intake of different foods in different age groups: young infants (< 6 months) are usually fed with breast milk, whereas a mixture of milk and food are consumed by older children (> 18 months). Therefore, infants of 6 ∼ 17 months were mainly consumer group of melamine-tainted milk powder and likely to be affected seriously.

The incidences of vomiting, diarrhea and fever were identified as important risk factor of melamine-associated stones in the study of Guan et al.10 In our study, diarrhea, fever, dehydration and URTI were all included as risk factors of melamine-associated AKI. These symptoms might cause oliguria, which could lead to a raise in the blood concentration of melamine and form obstructive stones and AKI and might even ever to influence the growth in the end. Accordingly, prophylaxis of these symptoms (diarrhea, fever, dehydration and URTI) in infants with melamine-associated urolithiasis is important; more water drank, even infusion, is necessary if these symptoms appear. Since vomiting might appear at any time during AKI, we have decided to exclude the symptom of vomiting to reduce bias in our study.

Nevertheless, there remained a significant difference between PIT of 6 ∼ 11 months and other periods, our regression model still rejected the PIT as a model-analysis risk factor, because of the main cause of AKI proved to be related to the total intake amount, but not the intake time. This result was perfect in accordance with the animal model studies by Puschner et al. and WHO.11,12

In our previously published studies,3,13 we found that melamine-induced urolithiasis could lead to AKI. Removing obstruction promptly by surgical intervention would be effective with satisfactory outcomes in our 18- and 36-month follow up. But for the risk factors of AKI, we did not draw a conclusion in the past studies. In our five-year follow-up study on these AKI children, the prophase results of some AKI children were trended to have low heights and weights possibly, which is accordance with others' follow-up studies.5 The probably reason is that ceasing milk intake for melamine-taint milk phenomenon, suffering AKI might have a negative influence on the children's important growth period or melamine might have some side-effects on children growth. Nevertheless, why it happened, we also should acknowledge different food intake, water and environments in different area should not be neglected.

We have attempted to identify and calculated all the risk factor concerned and the essence of the relationship amongst the risk factors and children growth in our studies. However, as many other retrospective and observational studies, our study has limitations, which need to be taken into account when considering its contributions. First is that selective bias is existed cannot be avoided. Second, it seems very likely that a study with long time periods would lost a number of follow-up data and generate the bias. Third, limited sample sizes and lacking control group in five-year follow-up would decrease the quality of our studies and could not be overlooked.

In summary, we could not reject that the possibility that our estimates were distorted because limitation above. However, this study indicates that high melamine concentration milk powder, young age (6 ∼ 17 months), as well as symptoms of URTI, diarrhea, dehydration or fever might be risk factors of AKI. The outcome of this melamine-associated AKI would be good if treat promptly. However, whether melamine or these risk factors have causality on effect of body growth of children still need further observation.

Declaration of interest

The authors declare no conflict of interest.