Differences in kidney function estimates based on creatinine and/or cystatin C in non-traumatic amputation patients and their impact on drug prescribing

Abstract

Accurate kidney function estimates are necessary when prescribing renally-eliminated medications. Our objectives were to investigate how amputation affects estimated glomerular filtration rate (eGFR) and to determine if dosing recommendations differ among different eGFR equations. In a cohort study of non-traumatic amputation patients, eGFR based on creatinine and/or cystatin C were measured before and after amputation. Prescribed, renally-eliminated medications were compared with dosing guidelines in Renbase®. Data from 38 patients with a median age of 75 years were analyzed. The median (range) eGFR was 65 (15–103), 38 (13–79), and 48 (13–86) mL/min/1.73 m2 before amputation and 80 (22–107), 51 (13–95), and 62 (16–100) mL/min/1.73 m2 after amputation for eGFRCreatinine, eGFRCystatinC, and eGFRCombined, respectively (p < 0.01). From before to after amputation, eGFR increased on average by 8.5, 6.1, and 7.4 mL/min/1.73 m2 for eGFRCreatinine, eGFRCystatinC, and eGFRCombined (all p < 0.01), respectively. At least one renally-eliminated medication was prescribed at a higher dose than recommended in 37.8% of patients using eGFRCystatinC, 17.6% using eGFRCombined and 10.8% using eGFRCreatinine. In conclusion, amputation affects eGFR regardless of the eGFR equations. The differences among equations would impact prescribing of renally-eliminated medications, particularly when switching from creatinine to cystatin C.