Combination of biomarkers for diagnosis of acute kidney injury after cardiopulmonary bypass

Figures & data

Table 1. Patient characteristics.

Variable	All	AKI (n = 25)	No AKI (n = 68)	p
Baseline
Age (y)	70 (61–76)	72 (67–78)	70 (61–75)	0.14
Female sex	31%	32%	30%	1.0
Pre-operative creatinine	91 (76–113)	86 (72–111)	95 (78–119)	0.29
Chronic kidney disease stage 3	29%	24%	31%	0.61
Chronic kidney disease stage 4	8%	16%	4%	0.08
Insulin-requiring diabetes	7%	8%	7%	1.0
Previous cardiac surgery	16%	20%	15%	0.53
L ventricular ejection fraction <35%	6%	16%	3%	0.04
Surgery
Coronary bypass grafts	59%	76%	53%	0.06
Valve replacement or repair	62%	68%	60%	0.63
Coronary bypass grafts + valve	25%	48%	16%	0.003
Thoracic aortic surgery	11%	20%	7%	0.13
Bypass time (min)	139 (11–202)	210 (146–240)	125 (106–177)	<0.001
Post-operative
APACHE III score	50 (41–57)	52 (47–69)	47 (40–56)	0.003
% Blood transfusion in theatre or first 24 h	41%	56%	35%	0.096
Any Vasopressor in first 24 h of ICU	48%	58%	44%	0.35
Any Inotrope in first 24 h of ICU	41%	44%	40%	0.81
Fluid balance (first 24 h post-op)	+190 (−886 to +1366)	+43 (−640 to +726)	+310 (−103 to +723)	0.73

Notes: Characteristics of 93 patients undergoing cardiopulmonary bypass by AKI-risk category. Continuous variables are expressed as medians (interquartile range), for categorical variables 95% confidence intervals are shown.

Table 2. ROC AUC analysis for a panel of biomarkers of AKI-risk.

Biomarker	No AKI n = 68	AKI n = 25	p (Univariate)	AUC	AUC 95% CI	Cut-off	Sens (%)	Spec (%)	p (AUC)
Pre-op
NGAL pre-op (ng/mL)	20.7 (5.7–68.3)	17.2 (6.7–62.5)	0.66
NGAL/Cr pre-op (ng/mg)	31.3 (8.4–100.4)	18.3 (5.3–40.7)	0.20
L-FAB pre-op (ng/mL)	1.0 (0.53–2.8)	2.1 (0.67–6.3)	0.09
α GST pre-op (ng/mL)	6.1 (2.4–11.8)	3.8 (1.7–7.5)	0.17
π GST pre-op (ng/mL)	9.4 (4.3–25.3)	11.4 (3.7–17.1)	0.63
Hepcidin pre-op (ng/mL)	481 (172–1053)	495 (132–961)	0.73
Hepcidin/Cr pre-op (ng/mg)	608 (295–1139)	427 (262–1008)	0.31
Serum CysC pre-op (mg/L)	1.2 (1.0–1.4)	1.2 (1.1–1.4)	0.51
Post-op
NGAL post-op (ng/mL)	135 (39–436)	383 (197–1632)	0.002*	0.71	0.59–0.83	>195	76.0	60.3	0.001*
NGAL/Cr post-op (ng/mg)	970 (312–2597)	4709 (1289–9105)	<0.001*	0.73	0.60–0.86	>2346	72.0	73.5	<0.001*
L-FAB post-op (ng/mL)	19 (3–75)	83 (18–143)	0.005	0.69	0.57–0.81	>9.5	88.0	41.2	0.003*
α GST post-op (ng/mL)	6.1 (2.6–11.5)	8.6 (3.6–25.2)	0.14	0.60	0.46–0.74	–	–	–	0.07
π GST post-op (ng/mL)	27.3 (6.6–67.7)	60.6 (148–336)	<0.001*	0.75	0.63–0.88	>71.6	72.0	77.9	<0.001*
Hepcidin post-op (ng/mL)	628 (214–1498)	698 (302–1470)	0.80	0.52	0.38–0.65	–	–	–	0.40
Hepcidin/Cr post-op (ng/mg)	5769 (2883–11,016)	3859 (2398–9971)	0.37	0.52	0.38–0.66	–	–	–	0.37
Serum CysC post-op (mg/L)	1.1 (1.1–1.3)	1.4 (1.2–1.5)	0.005	0.69	0.56–0.82	>1.24	76.0	63.2	0.003*
24 h
NGAL 24 h (ng/mL)	80 (35–159)	231 (63–399)	0.02	0.65	0.52–0.79	>207	52.0	82.4	0.011
NGAL/Cr 24 h (ng/mg)	79 (33–246)	68 (200–1206)	0.004	0.70	0.57–0.82	>126	64.0	69.1	0.002*
LFAB 24 h (ng/mL)	12.4 (6.8–24.8)	17.5 (6.1–42.2)	0.35	0.56	0.42–0.71	–	–	–	0.17
α GST 24 h (ng/mL)	7.0 (3.2–11.8)	5.2 (2.2–9.6)	0.26	0.57	0.44–0.71	–	–	–	0.13
π GST 24 h (ng/mL)	27.9 (11.7–54.4)	3.4 (15.6–62)	0.17	0.59	0.45–0.74	–	–	–	0.09
Hepcidin 24 h (ng/mL)	8581 (2610–13,492)	2881 (901–4815)	<0.001*	0.73	0.62–0.84	<7855	92.0	52.9	0.0004*
Hepcidn/Cr 24 h (ng/mg)	7935 (4484–11,061)	3846 (2650–5243)	<0.001*	0.77	0.67–0.86	<7313	96.0	57.4	<0.001*
Serum CysC 24 h (mg/L)	1.2 (1.1–1.5)	1.7 (1.3–2.1)	0.001*	0.72	0.59–0.85	>1.57	64.0	77.9	<0.001*

Notes: Prediction of AKI-risk in 93 patients in the first 5 days following CPB. p (Univariate): p value for univariate comparison AKI versus No AKI (Mann–Whitney U test). p (AUC): p value for ROC-AUC versus AUC = 0.5.

*Remained significant at p < 0.05 level after correction for 16 multiple comparisons (Bonferroni).

Figure 1. ROC curves for three AKI biomarker combinations with statistically significant increase in AUC. (Panel A) urine π-GST post-op and serum Cystatin-c at 24 h; (Panel B) urine π-GST post-op and urine Hepcidin:Creatinine at 24 h; (Panel C) urine NGAL:Creatinine post-op and urine Hepcidin:Creatinine at 24 h. We considered linear combinations of biomarkers in the form: C = B1 + α × B2 and determined the value of α (range −∞ to +∞) that provided maximal ROC-AUC for the ability of C to predict AKI. Only combinations of structural biomarkers (NGAL, π-GST) and filtered substances (Hepcidin, CysC) demonstrated statistical significance suggesting markers assessing different aspects of the pathophysiology of AKI may have greater combined diagnostic value.

Table 3. Linear combinations of biomarkers and decision tree analysis.

			Uncategorized NRI				IDI				Best cut-off			2-Category NRI
Combination	AUC	p	Total	p	Events	Non-events	Total	p	Events	Non-events	Value	Sens.	Spec.	Total	p	Events	Non-events
π-GST (post-op) − 0.03 × Hepcidin:Cr (24 h)	0.86	0.01	0.57	0.01	0.04	0.53	0.15	0.015	0.11	0.04	>−129	92%	69%	0.08	0.24	−0.04	0.12
NGAL:Cr(post-op) − 1.32 × Hepcidin:Cr (24 h)	0.84	0.03	0.45	0.05	0.04	0.41	0.09	0.06	0.07	0.02	>−5824	92%	66%	0.05	0.45	−0.04	0.09
π-GST (post-op) + 87.35 × CysC (24 hr)	0.83	0.03	1.02	<0.001	0.52	0.50	0.06	<0.001	0.05	0.02	>197	76%	79%	0.05	0.49	0.04	0.015
Classification and Regression Tree analysis: NGAL:Cr (post-op) > 7444 OR Hepcidin:Cr (24 h) < 5247												92%	71%	0.09	0.12	−0.04	0.13

Note: Biomarkers assessed by ability of ROC best cut-off to classify high or low risk of AKI, reclassification compared to better of the two individual markers.

Figure 2. Classification and Regression Tree (CART) analysis for biomarkers in combination. All biomarkers were included but only NGAL:Cr post-op and Hepcidin:Cr at 24 h remained in the final model. Terminal nodes identified high, medium and low risk groups for AKI. This model allows early identification of a high-risk group immediately post-operatively and then a low risk classification at 24 h.