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Assessment of kidney function for drug dosing.

The effort to improve the identification and management of patients with chronic kidney disease (CKD)1 is based on implementing more precise means of assessing kidney function and kidney damage in the clinical setting. Over the past decade, the clinical chemistry community has adopted routine reporting of estimated GFR (eGFR) calculated from the Modification of Diet in Renal Disease (MDRD) Study equation and is implementing a creatinine standardization program to enable the manufacturers of laboratory methods to establish calibration traceability to an isotope-dilution mass spectrometry (IDMS) reference measurement procedure. As a result, routine reporting of eGFR for adults based on an IDMS-traceable creatinine is becoming the clinical standard for patient care (1)(2).

As with all changes in clinical practice, the implementation of eGFR reporting and creatinine standardization has created some uncertainty and confusion among health care providers. A focus of concern is the assessment of kidney function for drug dosing adjustment. Standardizing creatinine calibration to an IDMS reference produces a lowering of creatinine values by 10%–20% for most methods. Pharmaceutical manufacturers have used the Cockcroft–Gault (CG) equation to estimate creatinine clearance as the basis for drug dose adjustment recommendations, and there is no modified equation available for use with the IDMS-traceable creatinine results. Consequently, creatinine clearance estimated from the CG equation will be erroneously high. Because eGFR using the MDRD Study equation is relatively new, eGFR data is not part of drug safety information or package inserts approved by the US Food and Drug Administration (FDA). Clinicians are most concerned about dosing highly toxic drugs with narrow therapeutic indices, particularly carboplatin, an antineoplastic agent.

Some clinical chemists have experienced resistance to creatinine standardization from clinicians in their facilities based on their uncertainty about how standardization would affect their ability to dose …