Abstract Background Increases in serum creatinine (ΔSCr) from baseline signify acute kidney injury (AKI) but offer little granular information regarding its characteristics. The 10th Consensus Conference of the Acute Dialysis Quality Initiative (ADQI) suggested that combining AKI biomarkers would provide better precision for AKI course prognostication. Objectives This study investigated the value of combining a functional damage biomarker (plasma cystatin C pCysC) with a tubular damage biomarker (urine neutrophil gelatinase-associated lipocalin uNGAL), forming a composite biomarker for prediction of discrete characteristics of AKI. Methods Data from 345 children after cardiopulmonary bypass (CPB) were analyzed. Severe AKI was defined as Kidney Disease Global Outcomes Initiative stages 2 to 3 (≥100% ΔSCr) within 7 days of CPB. Persistent AKI lasted >2 days. SCr in reversible AKI returned to baseline ≤48 h after CPB. The composite of uNGAL (>200 ng/mg urine Cr = positive +) and pCysC (>0.8 mg/l = positive +), uNGAL+/pCysC+, measured 2 h after CPB initiation, was compared to ΔSCr increases of ≥50% for correlation with AKI characteristics by using predictive probabilities, likelihood ratios (LR), and area under the curve receiver operating curve (AUC-ROC) values. Results Severe AKI occurred in 18% of patients. The composite uNGAL+/pCysC+ demonstrated a greater likelihood than ΔSCr for severe AKI (+LR: 34.2 13.0:94.0 vs. 3.8 1.9:7.2) and persistent AKI (+LR: 15.6 8.8:27.5 versus 4.5 2.3:8.8). In AKI patients, the uNGAL−/pCysC+ composite was superior to ΔSCr for prediction of transient AKI. Biomarker composites carried greater probability for specific outcomes than ΔSCr strata. Conclusions Composites of functional and tubular damage biomarkers are superior to ΔSCr for predicting discrete characteristics of AKI.