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THERAPEUTIC MONITORING

New Serum Creatinine Assay Standardization: Implications for Drug Dosing

Professor, Department of Clinical and Administrative Pharmacy, College of Pharmacy, University of Georgia, Athens, GA

William J Spruill, PharmD FASHP FCCP

Professor, Department of Clinical and Administrative Pharmacy, College of Pharmacy, University of Georgia

Reprints: Dr. Wade, Department of Clinical and Administrative Pharmacy, College of Pharmacy, University of Georgia, Athens, GA 30602, fax 706/542-5228, bwade{at}mail.rx.uga.edu

OBJECTIVE: To describe the development of new serum creatinine assay standards and discuss their implications for various drug dosing scenarios.

DATA SOURCES: A MEDLINE search (1990-August 2006) was performed to identify literature on newly developed serum creatinine assay standards and the impact those standards will have on drug dosing.

STUDY SELECTION AND DATA EXTRACTION: Data on the development of new creatinine assay procedures and the effect that these new values will have on drug dosing were extracted from identified references.

DATA SYNTHESIS: The National Kidney Foundation's current practice guidelines recommend standardization of serum creatinine assay calibration to increase assay accuracy. This will result in a lower than the current range of values being considered normal, as well as lower value patient values. These lower creatinine values will result in higher calculated glomerular filtration rates and creatinine clearance (Cl_cr) estimates, resulting in the difficulty of clinicians applying these values to current manufacturers' recommended drug dose modification tables in patients with declining renal function. Newer creatinine values may be as much as 5-20% lower than older assay values.

CONCLUSIONS: Worldwide implementation of new serum creatinine calibration standards will produce more accurate serum creatinine values. These values may be slightly lower than those obtained with older methods. This, in turn, may cause slightly higher Cl_cr estimates that may no longer match drug manufacturers' data for dosage adjustment in renal insufficiency, given that manufacturers' dosing tables were based on older creatinine values. If this is deemed clinically significant in a specific patient, increasing the creatinine value obtained with new assay techniques by 5-20% may give the clinician a value that more closely approximates that used by the drug manufacturer in developing these tables. This revised value can then be used for dosage adjustments.

Key Words: assay, creatinine, drug dosing, renal impairment

Published Online, March 6, 2007. www.theannals.com, DOI 10.1345/aph.1H586

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