THE RENAL PHOSPHATE THRESHOLD DECREASES WITH INCREASING POSTMENSTRUALAGE IN VERY-LOW-BIRTH-WEIGHT INFANTS

Our objective was the study of the renal phosphate threshold (T-P/GFR) in very low birth weight infants with increasing postmenstrual (pm) a ge (gestational age plus postnatal age). The case notes of 62 very low birth weight infants were reviewed. Plasma and urine phosphate concen trations (P-P, U-P) determined on the same day together with the corre sponding creatinine concentrations (P-Crea, U-Crea) built up a data se t. Data sets obtained from 29 to 36 wk of pm age were included in the study. U-P greater than or equal to 1 mmol/L was defined as phosphatur ia. T-P/GFR = P-P - (U-P x P-Crea/U-Crea). In infants without phosphat uria, maximum P-P is a lower limit of T-P/GFR and was used as a censor ed T-P/GFR value. We found that in phosphaturic infants, maximum P-P ( median and range) decreased from 2.8 (1.2-4.6) to 2.0 (1.4-2.7) mmol/L from 29-30 to 35-36 wk of pm age (p < 0.001), and censored T-P/GFR (m edian and 95% confidence interval) decreased from 2.13 (1.95-2.33) to 1.57 (1.31-1.77) mmol/L (p < 0.001). We speculate that the renal phosp hate threshold declines with increasing postmenstrual age because tubu lar reabsorption capacity increases more slowly than GFR.