Despite methodologic problems, endogenous creatinine clearance is commonly used as an estimation of glomerular filtration rate (GFR). Inulin clearance was compared to endogenous creatinine clearance in a group of low birthweight infants to establish the validity of the latter. Thirty-three low birthweight infants (birthweight mean = 1600 g, gestational age mean = 33 wk) were studied between 10 hr and 10 days of age to simultaneously measure GFR by inulin and endogenous creatinine clearances. Inulin and creatinine clearances correlated directly (r = 0.738, P > 0.001). The slope of the regression line suggested an overestimation of GFR (inulin clearance) by creatinine clearance at the low GFR range and an underestimation at the high GFR range. The data were divided into two groups by the median inulin clearance (12.5 ml/min/1.73m2). The ratio of creatinine to inulin clearance was significantly higher in the low GFR group (1.28 ± 0.16 vs. 0.89 ± 0.04 SEM, n = 19, P < 0.05). There was no difference between the two groups in plasma creatinine, birthweight, gestational age, incidence of respiratory distress, or oxygen requirements at the time of the studies. Endogenous creatinine clearance represents a good estimation of GFR (inulin clearance) in low birthweight infants. However, at the low GFR range, it represents an overestimation and at the high GFR range, an underestimation.
In low birthweight infants, tubular secretion of creatinine and plasma chromogens interfere with the accuracy of endogenous creatinine clearance. Tubular secretion of creatinine is probably of relatively greater importance at lower GFR's accounting for an overestimation of GFR by creatinine clearance. At higher GFR's, plasma chromogens are probably of more importance and account, in part, for the underestimation of GFR by creatinine clearance.