Renal handling of beta-2-microglobulin in the human neonate

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Glomerulotubular balance for beta-2-microglobulin (beta 2M) in the human kidney has been reported to occur after 34 weeks conceptional age (CA), and fractional tubular reabsorption of beta 2M (T beta 2M) has been suggested as a useful index of renal tubular maturation. To confirm and extend these observations to include still less mature infants, renal handling of beta 2M was investigated during timed-urine collections with corresponding blood samples obtained from 57 infants with CA of 26 to 43 weeks and postnatal ages (PNA) 0.2 to 12 days (study 1); 18 infants were studied a second time 5 to 17 days later (study 2). GFR was measured by endogenous creatinine clearance (CCr). T beta 2M and fractional reabsorption of sodium (TNa) were calculated. Results indicated that while both increased with CA, T beta 2M (r = -0.69, P less than 0.0001) and TNa (r = -0.79, P less than 0.0001) varied inversely with fractional urine flow rate (V/CCr). Moreover, an inverse relationship between changes in T beta 2M and V/CCr was observed in the same infant between study 1 and study 2 (r = -0.47, P less than 0.05). These data suggest that the renal handling of beta 2M in the human neonate is influenced by physiologic variables that are independent of CA, and thus T beta 2M may not be a reliable predictor of renal tubular maturation in the human neonate.