Jing Ng

Abstract

Background Acute kidney injury (AKI) is a common complication in hospitalized patients and is associated with a high mortality rate. A precise estimation of creatinine clearance (Clcr) is of great importance in daily clinical practice, particularly among critically ill patients with unstable kidney function, in guiding drug dosage adjustment. Clcr can be determined precisely through a simple laboratory method (amount of total urinary creatinine divided by the average of two consecutive serum creatinine concentrations) in patients with stable kidney function. However, in patients with rapidly improving kidney function, Clcr determined by the laboratory method can produce highly variable and inaccurate results due to the non-linear changes in plasma creatinine concentrations. Aim To determine the magnitude of discrepancies of Clcr calculated using the two-point average method to the actual Clcr and derive a correction factor for creatinine clearance determined by the two-point average method. Methods Patients with improving kidney functions were simulated. The actual urinary creatinine clearance, Ae 0-24, was subsequently calculated using area under the curve (AUC) 0-24 hours via Simpson’s approximation. Creatinine clearances were then calculated using Ae 0-24 / AUC0-24 and the 2-point average method. Results When Clcr improved greater than 35 ml/min, Clcr estimated by the 2-points method deviated significantly in progress from the actual Clcr whereby it underestimated the Clcr by 38%. The derived correction factor is 65 • [Clcr(ne)/ 33]. The correlation between actual Clcr and Clcr with correction factor was reported as R2 = 0.9882. Conclusion During the improvement kidney function, measuring urinary creatinine clearance using the 2-points average method can produce highly inaccurate Clcr measurement, particularly at Clcr ≥ 35 ml/min. The correction factor derived is found to be able to address the mentioned pitfall with low error.