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Introduction: The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula has not been validated in patients with cancer. The present investigation was undertaken in order to study how well estimated glomerular filtration rate (eGFR) using the new CKD-EPI equation correlates with measured GFR (mGFR) by (51)chrome ethylene diamine tetraacetic acid ((51)Cr-EDTA) clearance in a group of patients with cancer not known as having cancer., Material and Methods: We investigated 185 patients with cancer who were referred for isotope measurement of GFR with (51)Cr-EDTA before initiating chemotherapy treatment. The agreement between CKD-EPI and (51)Cr-EDTA was assessed using a Bland-Altman plot. Test performance was analysed in a contingency table and bias, precision and the percentage of estimates within 30% of the mGFR (P30) were assessed., Results: Bland-Altman plot analysis showed a limit of agreement in the range from -25.59 to 27.92 ml/min./1.73 m(2). This formula was therefore not interchangeable with (51)Cr-EDTA, as the above differences are of clinical importance. Bias was low: 1.16 ml/min./1.73 m(2); P30 was high: 89.73%; and precision was 13.37 ml/min./1.73 m(2). As a screening test, the CKD-EPI had a high specificity of 98% (95% confidence interval (CI): 96 to 100%) and a high negative predictive value 97% (95% CI: 95 to 100%). The accuracy of the validation test was 96% (95% CI: 93 to 99%)., Conclusion: The CKD-EPI may be used as a screening tool for CKD in the general population, but cannot replace isotope tests when a high GFR measurement accuracy is needed.