Better renal-function estimates not expected to alter drug dosing right away

An effort to have laboratories more accurately measure serum creatinine levels and estimate glomerular filtration rate (GFR), an index of kidney function, will not immediately change how pharmacists adjust the dosages of drugs for patients with renal impairment, according to two authors of textbooks on pharmacokinetics. William J. Spruill, a coauthor of ASHP’s Concepts in Clinical Pharmacokinetics, pointed to the pharmacokinetics equations widely used by pharmacists and the dosage-adjustment instructions in FDA-approved product labeling. He offered for illustration a popular equation for approximating the elimination rate constant of gentamicin in adults, 0.00293 × (creatinine clearance) + 0.014. Automatically substituting an estimate of GFR for the estimate of creatinine clearance would be “putting what could or could not be a better value into an equation that’s not prepared to accept it,” Spruill said. No one has validated the use of GFR estimates in that easy-to-use equation, he said. Nor does he expect anyone to develop and validate an equation that uses a GFR estimate to approximate gentamicin’s elimination rate constant.