Don't use estimated GFR to renally dose all medications

We used to just use the Cockcroft-Gault equation to calculate CrCl for renal dosing. But there's an increase in using eGFR calculated by MDRD....
Abdelwahab Ward, BS Pharm, PharmD

The debate continues about whether to use creatinine clearance (CrCl) or estimated glomerular filtration rate (eGFR) for renal dosing. We used to just use the Cockcroft-Gault equation to calculate CrCl for renal dosing. But there's an increase in using eGFR calculated by the Modification of Diet in Renal Disease (MDRD) equation, such as in dosing recommendations for canagliflozin (Invokana) and sacubitril/valsartan (Entresto).

     Keep in mind that CrCl and eGFR are NOT interchangeable. For example, a patient can have a CrCl of 26 mL/min and an eGFR of 38 mL/min/1.73 m2. If a medication is renally adjusted for a CrCl under 30 mL/min, you'll likely miss it using eGFR. In fact, you may miss adjusting dosing up to 50% of the time if you use eGFR when you should be using CrCl, potentially increasing adverse effects.

Follow package labeling recommendations when renally dosing medications. For example, labeling recommends renally adjusting apixaban (Eliquis) using CrCl. But evaluate eGFR to determine if a patient with renal dysfunction can be started on metformin. If labeling doesn't specify whether to use CrCl or eGFR, look at the units, use CrCl for mL/min AND eGFR for mL/min/1.73 m2. But don't only rely on the numbers.. they're just estimates. Use your judgment and incorporate urine output, illness severity, etc. For example, you may want to adjust a medication in a patient with a rising creatinine and no urine output, even if their CrCl is above the cutoff. But consider waiting 24 hours to renally adjust antibiotics in a septic patient with acute renal failure. 

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Abdelwahab Ward, BS Pharm, PharmD