You are asked to see a patient in acute renal failure. You note that his creatinine has increased from 100 to 200 umol/L in one day.
Why is it inappropriate to estimate their GFR using an estimation formula (such as Cockcroft-Gault or MDRD) by inserting 200 umol/L as the value for creatinine?
The eGFR estimation formulas were derived using patients at steady state, whose renal function was stable and can only be used in patients with stable creatinines.
Imagine a patient with a baseline creatinine of 100 umol/L. If this patient’s renal arteries are both clamped, his GFR will suddenly fall to 0, yet his creatinine will still be 100 umol/L. The next day his creatinine may have risen to 200 umol/L, yet his GFR would still be 0 ml/min. This example shows how the GFR cannot be determined using an isolated creatinine value in the setting of acute renal failure.