Formula for Kidney Function Measurement in Children Revised

A more precise, revised formula for measuring kidney function in children with chronic kidney disease was evaluated.

Glomerular filtration rate (GFR), the flow rate of fluid through the tiny capillaries in the kidney (glomeruli) that filter waste materials out of the blood and into the urinary system, is the most useful indicator of kidney function. However, determining true GFR is costly, time-consuming, and difficult to perform at regular clinical office visits.

The Schwartz formula, developed in the 1970s, estimates GFR from an equation that uses serum creatinine (a waste product in the blood) and height. A chronic kidney disease in children (CKiD) study was led by Prof. George Schwartz from University of Rochester (NY, USA). One of the enrollment criteria for the study, which assessed children with mild to moderate chronic kidney disease, was the glomerular filtration rate (GFR) estimated by the Schwartz formula. Investigators compared the Schwartz formula to a Scandinavian method of measuring GFR using disappearance of the X-ray dye iohexol from the blood. The iohexol method was only used every two years after the first two visits in the CKiD trial, and an accurate assessment of GFR was needed during the annual visits when the iohexol method was not performed.

After comparing various estimated GFR models using a testing data set of 168 children in the CKiD trial, the investigators found that a modified Schwartz formula using height, serum creatinine, cystatin C (a small protein expressed throughout the body), blood urea nitrogen, and gender came the closest to replicating the results of the iohexol method, widely regarded as the gold standard in measuring GFR.

"The relationship between estimated GFR and the biochemical markers may be different in this population than in a population with more normal kidney function and without poor skeletal growth,” said Prof. Schwartz. "Although we did not observe that our formula changed with puberty, other populations with more normal physique and health status should be examined to evaluate the estimated GFR for adolescents.”

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