Things that bug me 2 – lack of understanding eGFR

Once upon a time, we had to estimate GFR from the creatinine using an app or even a calculator.  Now almost all labs provide estimated GFR, but this is both a positive and a negative. I rarely find a resident (or attending physician) who understands the principles behind eGFR, and thus too often they interpret the number incorrectly and use it wrong.  Here is my quick guide for understanding eGFR. Back in the day, before the turn of the century, we measured 24 hour urines.  We would measure the volume over 1440 minutes and measure the urine creatinine level.  Given these numbers and the serum creatinine level, we would plug the numbers into the classic formula UV/P. This method had the major problem of actually collecting an accurate 24 hour urine. Cockroft and Gault published this article in 1976 – Cockcroft, D.W. and M.H. Gault. Prediction of creatinine clearance from serum creatinine. Nephron. 1976. 16(1):31-41. The formula Creatinine clearance = (140-age)*wt kg / Scr * 72 for men and 85% of that number for women was the commonly used for many years. Two other formulas based on the same reasoning are likely familiar to many readers – the MDRD formula (used by most labs) and the CKD-EPI equation. What are these formulas all doing?  They are estimating daily creatinine production!  The numerator of the creatinine clearance formula, while the creatinine is stable, represent daily creatinine production.  In steady state situations, virtually all the creati...
Source: DB's Medical Rants - Category: Internal Medicine Authors: Tags: Medical Rants Source Type: blogs