Glomerulotubular balance for beta-2-microglobulin β2M in the human kidney has been reported to occur after 34 weeks conceptional age (CA), and fractional tubular reabsorption of β2M (T(β2M)) 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 β2M 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 (C(Cr)). T(β2M) and fractional reabsorption of sodium (T(Na)) were calculated. Results indicated that while both increased with CA, T(β2M) (r = -0.69, P < 0.0001) and T(Na) (r = -0.79, P < 0.0001) varied inversely with fractional urine flow rate (V/C(Cr)) was observed in the same infant between study 1 and study 2 (r = -0.47, P < 0.05). These data suggest that the renal handling of β2M in the human neonate is influenced by physiologic variables that are independent of CA, and thus T(β2M) may not be a reliable predictor of renal tubular maturation in the human neonate.
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