Differences in Kidney Function Estimates Based on Creatinine and/or Cystatin C in Non-Traumatic Amputation Patients and Their Impact on Drug Prescribing

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((R)). 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 m(2) before amputation and 80 (22-107), 51 (13-95), and 62 (16-100) mL/min/1.73 m(2) after amputation for eGFR(Creatinine), eGFR(CystatinC), and eGFR(Combined), 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 m(2) for eGFR(Creatinine), eGFR(CystatinC), and eGFR(Combined) (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 eGFR(CystatinC), 17.6% using eGFR(Combined) and 10.8% using eGFR(Creatinine). 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.