Updated height- and creatinine-based equation and its validation for estimation of glomerular filtration rate in...

Since the original Schwartz formula overestimates glomerular filtration rate (GFR), it is proposed that the constant (k) that accounts for the method of creatinine estimation be derived locally. We derived a new k for height (cm)/serum creatinine (mg/dl) (ht/scr) equation by regression analysis.

In a cross-sectional observational study, 197 children (2-18 years) with chronic kidney disease (CKD), who underwent reference GFR measurement by plasma clearance of diethylenetriamine pentaacetic acid (dGFR) at a tertiary care hospital, formed the index dataset for deriving the prediction equations for estimating GFR. Serum creatinine was estimated by the kinetic Jaffe method. The prediction equations were validated on a separate cohort of 225 children with CKD.

The median creatinine was 0.7 mg/dl and dGFR was 80.5 (interquartile range 18.1-137.5) ml/min/1.73 m(2). The new k (regression coefficient of height/creatinine) was 0.42 (R(2) = 0.61) and the updated equation was GFR = 0.42 × (ht/scr). Addition of age and mid-arm circumference (MAC) to this equation improved R (2) to 62.3%. Based on the above parameters, the new equation for estimating GFR was GFR (ml/min/1.73 m(2)) = 0.257 × [ht/scr](0.95) × [age (year)](-0.19) × [MAC (cm)](0.397). The two equations performed comparably, with a mean bias <2 ml/min/1.73 m(2). The updated ht/scr equation yielded 74% and 24% estimated GFR values that were within 30% and 10% of the measured dGFR, respectively.

The updated equation, with a k value of 0.42, provides a reasonably accurate bedside estimate of GFR in children in countries where creatinine is estimated by the kinetic Jaffe method.