Determination of a new constant to estimate glomerular filtration rate in pediatrics

Introduction. In pediatrics, glomerular filtration rate (GFR) may be estimated by measured corrected creatinine clearance ( mcCrCl) (mL/min/1.73 m(2)) or the Schwartz formula (eGFR = height/plasma creatinine x k). The constant k depends on the plasma creatinine determination method: k = 0.55 for the Jaffe colorimetric method and k = 0.413 for the enzymatic method. Our laboratory uses the compensated kinetic colorimetric assay (CKC), and differences are observed between the estimated and measured GFR. Hypothesis: The proposed values of k do not adjust to the CKC method for plasma creatinine. Objective. To calculate a k value that allows to estimate GFR through creatinine measurement with CKC. Methods. Correlational, descriptive design. Patients aged 3-18 years seen at the Division of Pediatric Nephrology between July 2017 and January 2018with normal oraltered GFR, bladder and bowel control, and signed consent were included. Malnourished and myelomeningocele patients were excluded. Studied variables were plasma and urine creatinine, height, and 24-hour urine output. Results. A total of 184 patients were analyzed, their mean age was 10 years. Median mcCrCl was 123 mL/min/1.73 m(2). The linear correlation between height and plasma creatinine and mcCrCl resulted in a k value of 0.499 (r = 0.974 and r(2) =0.949). The linear correlation between the estimated GFR (k = 0.499) and mcCrCl resulted in a 0.999 beta coefficient (r = 0.951 and r(2) = 0.903). Conclusion. According to this study, the constant that allows to estimate GFR when measuring plasma creatinine with the CKC method is 0.499.