The renal phosphate threshold decreases with increasing postmenstrual age in very low birth weight infants.

Our objective was the study of the renal phosphate threshold (TP/GFR) in very low birth weight infants with increasing postmenstrual (pm) age (gestational age plus postnatal age). The case notes of 62 very low birth weight infants were reviewed. Plasma and urine phosphate concentrations (PP, UP) determined on the same day together with the corresponding creatinine concentrations (PCrea, UCrea) built up a data set. Data sets obtained from 29 to 36 wk of pm age were included in the study. UP > or = 1 mmol/L was defined as phosphaturia. TP/GFR = PP - (UP x PCrea/ UCrea). In infants without phosphaturia, maximum PP is a lower limit of TP/GFR and was used as a censored TP/GFR value. We found that in phosphaturic infants, maximum PP (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 TP/GFR (median 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 phosphate threshold declines with increasing postmenstrual age because tubular reabsorption capacity increases more slowly than GFR.