PUERTO RICO—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..

“We need to develop a better real-time indication of kidney function in critically ill patients, especially those with AKI,” said lead investigator Erin Frazee, PharmD, RPh, of Hospital Pharmacy Services at Mayo Clinic in Rochester, Minn. “Because if you misinterpret GFR, you’re likely to give patients doses of medication which may be dangerously high or low, and potentially expose them to nephrotoxins which may further exacerbate the process.”

She and her colleagues retrospectively studied adults treated at their center from January 2006 to June 2011. They focused on individuals who met the Acute Kidney Injury Network (AKIN) urine-output criterion for stage 3 AKI after at least two days in the ICU. The investigators excluded patients who developed AKI or required dialysis within less than 48 hours of ICU admission; did not have a urinary catheter; or who had a history of end-stage renal disease or kidney transplantation.

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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).