Fluid overload occurs early after cardiac surgery and is associated with prolonged PICU length of stay and ventilation. Future fluid overload avoidance trials may confirm or refute a true fluid overload-outcome causative association.

Background Early accurate acute kidney injury (AKI) diagnosis is needed to pursue AKI treatment trials. We evaluated Cystatin C (CysC) as an early biomarker of serum creatinine (SCr)-AKI and as an alternative to define AKI. Methods 160 non-cardiac children admitted to our intensive care unit (ICU) were prospectively studied. We measured daily CysC and SCr. AKI was staged according to the Kidney Disease: Improving Global Outcomes (KDIGO) SCr criteria and by similarly applied criteria, using CysC (CysC-AKI) We calculated area under the curve (AUC) for 1) neutrophil gelatinase associated lipocalin (NGAL), interleukin-18 (IL-18), kidney injury molecule-1 (KIM-1) and urine cystatin C (uCysC) to diagnose SCr- and CysC-AKI and 2) for CysC to diagnose SCr-AKI. We evaluated the relation of each AKI definition with length of stay and mechanical ventilation duration. Results 44% of patients developed SCr-AKI; 32% developed CysC-AKI. Whether AKI was defined by SCr or CysC, NGALwas associated with AKI severity. CysC-AKI was not more strongly associated with clinical outcomes. However, early ICU admission CysC predicted SCr-AKI development within 48 hours (AUC=0.70 [95% CI 0.53 – 0.89]). Conclusions Our findings do not support replacing SCr by CysC to define AKI. However, CysC may be used for predicting AKI development when obtained early in ICU admission.