Comparison of methods of estimating creatinine clearance in pediatric patients

Purpose. A retrospective study was conducted to compare various methods of measuring serum creatinine (SCr) values for use in pediatric renal function assessments, including a method aligned with a recently implemented national SCr testing standard. Methods. Demographic, medication-use, and selected laboratory data were collected from the hospital records of a sample of pediatric patients (n = 91) who underwent 12- or 24-hour timed urine collection for determination of creatinine clearance (CLcr) over a 2-year period. Documented CLcr values measured via the timed urine collection method were compared with investigator-calculated estimates of CLcr or glomerular filtration rate (GFR) derived using 3 SCr-based methods: the Counahan-Barratt equation; the original Schwartz equation; and the "bedside IDMS-traceable Schwartz equation," a modified version of the Schwartz equation reflecting the recent shift toward isotope dilution mass spectrometry (IDMS) methods of SCr measurement, which have been found to yield SCr values 10-20% lower than those derived by older methods, potentially resulting in GFR overestimation if traditional formulas for estimating GFR are used. Results. Comparisons of timed urine collection-derived CLcr values with CLcr values derived from the 3 comparator equations indicated significant levels of bias in all cases, with calculated correlation coefficients of 0.71 for the original Schwartz equation, 0.72 for the bedside IDMS-traceable Schwartz equation, and 0.72 for the Counahan-Barratt equation. Conclusion. Pediatric CLcr values calculated using the original Schwartz, bedside IDMS-traceable Schwartz, and Counahan-Barratt equations were well correlated, but none of the 3 equations yielded values that correlated well with CLcr values derived via the gold-standard method of timed urine collection.