Expert offers these guidelines for measuring renal function

Hospital pharmacists have a variety of options now for the estimation of renal function for the purpose of drug dose adjustment, and it's sometimes difficult to decide the best approach.

An expert offers some suggestions and best practice guidelines for making this determination.

Gary R. Matzke, PharmD, FCP, FCCP, FASN, DPNAP, professor of pharmacy and pharmaceutics and professor of medicine at the Medical College of Virginia, Virginia Commonwealth University in Richmond, VA, spoke about methods for estimating renal function in a talk at the 44th American Society of Health-System Pharmacists (ASHP) clinical meeting and exhibition, held Dec. 6-10, 2010, in Las Vegas, NV. The following suggestions are based on his talk and slide presentation on the topic:

• Follow these learning objectives:

- Pick the best marker of renal function on the basis of the component it measures;

- The accuracy and limitations of the various procedures that can be utilized to measure GFR are considerable, but one should measure when estimation methods yield divergent results;

- Know strengths and weaknesses of the various methodologies for estimating CrCl or GFR, and choose the best method for the population of patients you predominantly care for.

• Review these practical considerations for adjusting drug doses:

- GFR has historically been expressed as mL/min/1.73 m2 body surface area (BSA). The MDRD equation is designed to report GFR in these same units.

- The original Cockcroft-Gault equation used the patient's total body weight (TBW) without additional BSA adjustments and yielded units of mL/min.

- Over time, this equation has evolved to replace TBW with either ideal body weight (IBW) or an adjusted body weight.

- Although the units with the C&G method are different than GFR, if one uses IBW instead of TBW no additional adjustments are necessary.

• Here are some considerations with MDRD:

- Make sure you use the right equation based on the methodology the hospital laboratory uses to measure serum creatinine;

- Most labs report "eGFR" values only when eGFR < 60 mL/min/1.73 m2 (otherwise reported as "GFR > 60 mL/min/1.73 m2);

- FDA labeling for most drugs now gives renal drug dosing recommendations based on CrCl (Cockcroft-Gault);

- Studies evaluating relationship between MDRD (eGFR) and total drug clearance currently are rarely available.

• Refer to these best practice recommendations:

- MDRD eGFR (4-variable) or CKD-EPI should be limited to CKD classification for the present time;

- CrCl using Cockcroft-Gault (mL/min) should be used for drug dosing algorithms;

- Accurately timed measured 24-hour CrCl, or estimated CG with weight index specified, should be used to stratify patients into renal pharmacokinetic studies;

- Prospective pharmacokinetic studies should identify the relationship between MDRD eGFR, as well as C&G estimated CrCl and drug clearance.