Take a closer look when assessing renal function
Prediction should be adjusted based on muscle mass
Two men are admitted to the hospital. One is a bodybuilder and looks like Arnold Schwarzenegger. The other is obese and describes himself as a "couch potato." They have the same body weight. What is the best way to predict their creatinine clearance?
Muscle mass makes a difference, says John E. Murphy, PharmD, professor and head of the Department of Pharmacy Practice and Science at the College of Pharmacy at the University of Arizona-Tucson. For the best prediction, he advises pharmacists to look at the actual patient first instead of just saying, "Predict their creatinine clearance on ideal body weight or actual body weight."
Because the input side is the skeletal muscle mass, each of the men would have the same creatinine clearance prediction if a pharmacist used the lean body weight or actual weight, Murphy explains. At the same serum creatinine level, however, Arnold Schwarzenegger types would have a much higher creatinine clearance.
"If you have a patient who looks like Arnold and has a lot of muscle, I suggest using his actual body weight to predict creatinine clearance and maybe even assume it’s higher," Murphy says. "A patient who’s very obese and more like a couch potato won’t have as much muscle mass relative to his total weight. You might want to use ideal body weight for him or some adjustment in between if he has some degree of muscle increase with his increase in body weight."
Murphy made a presentation about the notion of muscle mass as an important factor in creatinine clearance estimates at the 2005 meeting of the American Pharmacists Association in April. His presentation was part of the "Clinical Pearls for Acute Care Pharmacists" section of the meeting.
The talk stemmed from a course Murphy teaches in clinical pharmacokinetic application. One of the subjects he tackles is, "What is the best way to make predictions of drug dosages and concentration outcomes?" His advice to assess the degree of a patient’s muscle mass is just a way to slightly improve predictions, he says.
"A lot of hospitals have dosing programs based on renal function, and pharmacists alter doses in patients with reduced function based on an estimated creatinine clearance," he says. "It would be good for them to take the opportunity to look at the patients and enhance their predictions."
Murphy doesn’t think that predicting creatinine clearance without considering a patient’s body makeup is necessarily harmful to patients. Estimating dose adjustments, however, might allow pharmacists to make more sophisticated decisions in their dosing approaches. He would also like to see more researchers make a distinction in their clinical results about which weight they used when predicting creatinine clearance so that clinicians can best adapt the research results to patient care.
Most overweight patients treated with the more general approach to predicting creatinine clearance would receive no more than about a 20% difference in dosing, Murphy says. "People aren’t going to go over the edge necessarily [with this difference], but you might refine your predictions and give them a better dose from the start."
Two men are admitted to the hospital. One is a bodybuilder and looks like Arnold Schwarzenegger. The other is obese and describes himself as a couch potato. They have the same body weight. What is the best way to predict their creatinine clearance?You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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