Drug dosing using estimated glomerular filtration rate: Misclassification due to metamizole interference in a creatinine assay

Abstract

Background: The estimated glomerular filtration rate is a rather important measurement for patients under intensive care, since they often receive several drugs, and impaired renal function may result in misleading dosing. The estimated glomerular filtration is derived from mathematical models using serum creatinine, a measurement that suffers interference of some drugs, such as metamizole. This study intended to evaluate the impact on patient stratification for dose adjustment of two antimicrobials (meropenem and vancomycin) caused by metamizole interference in creatinine measurement by dry chemistry.

Methods: A cross-sectional study was conducted with a group of 108 hospitalized patients under metamizole prescriptions at fixed intervals. Serum creatinine concentrations were determined by enzymatic dry chemistry and Jaffé assays, and the estimated glomerular filtration rate was calculated through the CKD-EPI equation. Patients were stratified in groups according to their estimated glomerular filtration rate for drug dosing of vancomycin and meropenem.

Results: Creatinine values were significantly lower in measurements performed by the dry chemistry method in comparison to Jaffé assay (P < 0.0001) when patients are under metamizole treatment. A significant bias (-40.3%) was observed between those two methods, leading to a significant difference (P < 0.0001) in patient classification according to renal function using the CKD-EPI equation for dosing adjustment.

Conclusions: During the validity of metamizole treatment, the stratification for drug dosing by the estimated glomerular filtration rate is not reliable if the creatinine measurement is done through dry chemistry. Clinical and laboratory staff must be aware of these limitations and cooperate to optimize pharmacotherapy.

Keywords: Creatinine; enzymatic methods; renal disease.