Worsening renal function (WRF) during treatment of acute decompensated heart failure (ADHF) is generally associated with adverse outcomes. An increase ≥0.3 mg/dL in creatinine level is widely used as the definition of WRF. The authors sought to determine the level of agreement between WRF based on changes in creatinine and changes in cystatin C (CysC) by analyzing data from 121 ADHF patients with available admission and day 3 creatinine and CysC levels. Admission creatinine and CysC levels were 1.39 (0.98-2.11) mg/dL and 1.95 (1.42-2.69) mg/L, respectively, and correlated well (r=0.81). On average, creatinine (-0.04±0.40 mg/dL) and CysC (0.001±0.34 mg/L) changed minimally from admission to day 3. Although the correlation between both markers on day 3 was still good (r=0.79), the correlation between changes therein was only modest (r=0.43). From the 14 and 15 patients who had WRF based on a ≥0.3 mg/dL increase in creatinine and ≥0.3 mg/L increase in CysC, respectively, only four (about 30%) met both definitions. These observations, together with recent insights in the inconsistencies of creatinine-defined concept of worsening renal function and outcomes, raises the need to research more reliable measures of renal function during treatment of ADHF.