Purpose Accurate estimation of renal function using glomerular filtration rate (eGFR) is key for prediction of adverse events among HF pts undergoing LVAD implant. Changes in muscle mass are frequent among HF pts and represent a known confounder of serum creatinine (sCr) based equations. Cystatin C (CysC) is independent of muscle mass and provides an alternative measure of eGFR. We aimed to prospectively compare the value of CysC vs. sCr based eGFR in the prediction of postoperative outcomes among pts undergoing LVAD implant. Methods sCr and CysC were concurrently measured pre-LVAD in 73 pts (age 61±13, F 16%). CKD stages were defined by sCr-eGFR and CysC-eGFR using MDRD4 and CysC-CKD-EPI equations, respectively. The primary endpoint was a composite of severe right ventricular failure (sRVF) or renal replacement therapy (RRT) on the index admission. Predictive values of sCr-eGFR and CysC-eGFR were compared. The independent effect of sCr and CysC on the primary endpoint was assessed using multivariable models. Results Compared to pre-LVAD sCr-eGFR, CysC-eGFR reclassified 46 (63%) pts: 7 (16%) to earlier stage CKD and 39 (84%) to later stage CKD (Fig A). For every increase in CKD stage, there was a 60% increase in the risk of the primary endpoint using CysC-eGFR (p=.02) while the increase in risk did not reach statistical significance using sCr-eGFR (p=.19) (Fig B). CysC-eGFR outperformed sCr-eGFR as predictor of the primary endpoint (AUC 0.68 vs. 0.59, p=0.04, Fig C). In a model including both CysC and sCr: i) higher CysC was associated with an increased risk for the primary endpoint (OR: 1.60 per 0.5 mg/L increase, p=.01); ii) a trend towards a paradoxical association between higher sCr and lower risk was found (OR: 0.42 per 0.5 mg/dL increase, p=.08). Conclusion CysC-eGFR is a more reliable predictor of sRVF or need for RRT among pts undergoing LVAD implant. Higher sCr in relation to CysC may represent a marker of preserved muscle mass and thus might paradoxically confer improved prognosis.