Background: Directly measuring kidney function is time-consuming, expensive and impractical. Thus different formulas have been developed for estimating the glomerular filtration rate (eGFR) which provides a surrogate marker of clearance. The MDRD equation is commonly used to provide eGFR to clinicians. However, most product information suggests prescribing based on the estimated creatinine clearance (eCrCL) based on the CockcroftGault (CG) formula. different equations utilise different factors in their equations, and thus have different accuracy in different patients groups. It the cohort of patients seen in the Mineral metabolism clinic, patients with increasing age and a lower weight tend to be common, compared to the average age of patients on which these equations were based. We thus set out to see how our prescribing would be influenced by different methods to estimate kidney function.
Methods: This study built on a previous pilot analysis. 100 patients given Zolendronic acid over 5 years in the mineral metabolism clinic were retrospectively selected and various parameters assessed. The eGFR was calculated using the MDRD equation and CKD EPI, and an eCrCl was calculated using the CG formula with or without modification for BMI (when available).
Results: 7 patients were found to have a eCrCl below the acceptable level for Zolendronic acid using the generic CG formula, and 9 using a Corrected eCrCl taking into account BMI. Some patients showed a temporary elevation of creatinine, but this normalised over time. In general the MDRD and CKD-EPI gave a higher eGFR, especially in older and lower BMI patients, compared to the CG formula.
Recommendations: Age and weight of individuals,as well as the strengths and limitations of different formulas, should be considered when prescribing medications in the mineral metabolism clinic. This may decrease the risk of nephrotoxicity and other side effects.