KEY POINTS: Studies on GFR assessment in people with cancer are needed. In this study, the creatinine-cystatin C equations performed better than other equations, and this was observed in solid and hematological cancers. Our findings give support to the preferential use of creatinine and cystatin C-based equations in people with cancer. BACKGROUND: GFR assessment is important in clinical practice with implications for diagnosis, prognostication, and drug dosing. People with cancer are at risk of imprecision in GFR estimation. This cross-sectional study evaluated the performance of various creatinine and cystatin C-based equations in comparison with measured GFR (mGFR) in people with cancer. METHODS: We retrieved data for all adult patients who had mGFR by urinary iothalamate clearance between 2011 and 2023 at Mayo Clinic and use of an electronic health record diagnosis code for cancer within 2 years before mGFR. The CKD Epidemiology Collaboration (CKD-EPI), European Kidney Function Consortium, and Cockcroft-Gault equations were computed, along with performance metrics (bias, precision, and root mean square error [RMSE]. Confidence intervals were generated by bootstrapping, and analysis were stratified by solid and hematological cancers. RESULTS: From all adults with cancer and mGFR, 1145 had both creatinine and cystatin C available within 7 days of mGFR. Among all equations, the creatinine-cystatin C CKD-EPI equation provided the best performance, with small bias (median, 3.0
95% confidence interval, 2.3 to 3.8) and higher precision (RMSE, 14.5) compared with creatinine-only or cystatin C-only equations (RMSE varying from 16.6 to 20), and this was also true in solid and hematological cancers. The creatinine-cystatin European Kidney Function Consortium equation had a similar performance to CKD-EPI, Cockcroft-Gault showed worst precision (30% of people with errors above 30%), and cystatin C CKD-EPI equation was the most biased, prone to underestimation of mGFR. CONCLUSIONS: In our cohort of patients with mGFR and cancer, the CKD-EPI creatinine-cystatin C equation performed best for GFR assessment, and this was true for both solid and hematological cancers. Our findings give support for the preferential use of creatinine and cystatin C-based equations instead of creatinine-only or cystatin C-only equations in people with cancer.