Assessing glomerular filtration rate (GFR) in critically ill patients with acute kidney injury - true GFR versus urinary creatinine clearance and estimating equations.

• INTRODUCTION: Estimation of kidney function in critically ill patients with acute kidney injury (AKI), is important for appropriate dosing of drugs and adjustment of therapeutic strategies, but challenging due to fluctuations in kidney function, creatinine metabolism and fluid balance. Data on the agreement between estimating and gold standard methods to assess glomerular filtration rate (GFR) in early AKI are lacking. We evaluated the agreement of urinary creatinine clearance (CrCl) and three commonly used estimating equations, the Cockcroft Gault (CG), the Modification of Diet in Renal Disease (MDRD) and the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations, in comparison to GFR measured by the infusion clearance of 51Cr-EDTA, in critically ill patients with early AKI after complicated cardiac surgery. METHODS: Thirty patients with early AKI were studied in the intensive care unit, 2 to 12 days after complicated cardiac surgery. Infusion clearance for Chromium-ethylenediaminetetraacetic acid (51Cr-EDTA) was obtained as a measure of GFR (GFR51Cr-EDTA) calculated from the formula; GFR (mL/min/1.73m2) = (51Cr-EDTA infusion rate x 1.73) / (arterial 51Cr-EDTA x BSA) and compared with the urinary CrCl and the estimated GFR (eGFR) from the three estimating equations. Urine was collected in two 30 min periods to measure urine flow and urine creatinine. Urinary CrCl was calculated from the formula; CrCl (mL/min/1.73m2) = (urine volume x urine creatinine x 1.73) / (serum creatinine x 30min x BSA). RESULTS: The within-group error was lower for GFR51Cr-EDTA than the urinary CrCl method, 7.2 %. vs. 55.0 %. The between-method bias was 2.6, 11.6, 11.1 and 7.39 ml/min, for eGFRCrCl, eGFRMDRD , eGFRCKD-EPI and eGFRCG , respectively, when compared to GFR51Cr-EDTA. The error was 103, 68.7, 67.7 and 68.0 % for eGFRCrCl, eGFRMDRD, eGFRCKD-EPI and eGFRCG, respectively when compared to GFR51Cr-EDTA. CONCLUSIONS: The study demonstrated a poor precision of the commonly utilized urinary CrCl method for assessment of GFR in critically ill patients with early AKI and should not be used as a reference method when validating new methods for assessing kidney function in this patient population. The commonly used estimating equations perform poorly, when estimating GFR, with high biases and unacceptably high errors.

Subject headings

MEDICIN OCH HÄLSOVETENSKAP  -- Klinisk medicin -- Anestesi och intensivvård (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Clinical Medicine -- Anesthesiology and Intensive Care (hsv//eng)

MEDICIN OCH HÄLSOVETENSKAP  -- Klinisk medicin -- Urologi och njurmedicin (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Clinical Medicine -- Urology and Nephrology (hsv//eng)

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