Blog entry by Meguid El Nahas
American Society for Critical Care Medicine (Puerto Rico): January 22, 2013
GFRs Overestimated in ICU Patients with AKI
Glomerular filtration rates (GFRs) of critically ill patients with acute kidney injury (AKI) are routinely overestimated, data presented at the Society for Critical Care Medicine's 2013 annual meeting suggest. Investigators believe urine output should be used instead of creatinine-based equations to assess kidney function in oligoanuric ICU patients.
The average baseline serum creatinine level was 0.9 mg/dL, and 10% of subjects had a documented history of chronic kidney disease. On each of the first four days of AKI, patients were between 1.8 and 3.7 liters fluid positive. Ten percent of the patients were prescribed trimethoprim.
The researchers assumed that the patients had a true GFR of less than 15 mL/min/1.73 m2. They compared this to the patients' estimated GFRs (eGFRs) calculated from six existing equations. The equations were the Cockcroft-Gault using actual body weight (CG-ABW), Cockcroft-Gault using ideal body weight (CG-IBW), Jeliffe, Modified Jeliffe, the four-variable Modification of Diet in Renal Disease (MDRD-4) study formula, and the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations.
Results of all six equations significantly overestimated GFR, even after the researchers adjusted for patients' daily variation in creatinine clearance. The closest approximation of the true GFR was given by the CG-IBW, which yielded a day-adjusted eGFR of 32 ml/min/1.73m2. The next-most accurate was the CG-ABW, with a day-adjusted eGFR of 51 ml/min/1.73m2. The least accurate was the Jeliffe equation, with a day-adjusted eGFR or 65 ml/min/1.73m2. Statistically and clinically significant overestimation of true GFR persisted out to the fourth day of AKI.
The findings echo those of previous studies. For example, a multicenter observational study published in 2010 showed the CG-ABW, MDRD and Jeliffe equations overestimated urinary creatinine clearance by 80%, 33% and 10%, respectively (Nephrol Dial Transplant 2010;25:102-107).
Clearly, this doesnt fully appreciate that:
1. eGFR (Regardless of the CR based Formula used) is NOT applicable to AKI!
2. eGFR is NOT applicable to non-steady state situations!
3. eGFR is NOT applicable to sick patients with malnutrition and sarcopenia!
4. Serum Creatinine is an UNRELIABLE marker of true GFR/Kidney Function in AKI!
Other Biomarkers are not much better and a circular argument goes that they rise before serum Cr goes up....but serum creatinine is an unreliable marker of AKI...so in the absence of Gold Biomarker for AKI, clinical judgement is key to the Diagnosis and Management of AKI!