Renal functional maturation: renal handling of proteins by mature and immature newborns.

Renal clearance of creatinine (Ccr), total protein excretion, urinary protein composition and renal clearance of albumin (Calb) were measured and calculated in male premature and mature infants of gestational age 29-41 weeks and in mature infants 1 and 3 months of age. Total protein excretion decreased slightly but not significantly during maturation. The urinary protein composition changed significantly as the fraction of low molecular weight proteins decreased from 48% at a gestational age of 29-33 weeks to 24% in mature infants aged 3 months, the albumin fraction increased from 39%-46% and the proportion of higher molecular weight proteins increased from 12%-29%, respectively. Calb decreased from 2.73-0.80 microliter/min/1.73 m2 in the presence of a rise in Ccr, resulting in a significant fall of the ratio Calb/Ccr from 0.0137 in the youngest prematures to 0.00147 in 3-month-old mature infants.