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The renal phosphate threshold decreases with increasing postmenstrual age in very low birth weight infants
W A Mihatsch1, R Muche, F Pohlandt
1Division of Neonatology and Paediatric Critical Care Medicine, Children's Hospital, Ulm, Germany.
Insights
The renal phosphate threshold declines in very low birth weight infants as they age. This indicates reduced phosphate reabsorption with increasing postmenstrual age.
Area of Science:
- Neonatalogy
- Pediatric Nephrology
- Biochemistry
Background:
- Very low birth weight infants often experience altered mineral metabolism.
- Understanding renal phosphate handling is crucial for their growth and development.
Purpose of the Study:
- To investigate the changes in renal phosphate threshold (TP/GFR) in very low birth weight infants.
- To correlate these changes with increasing postmenstrual age.
Main Methods:
- Retrospective review of 62 very low birth weight infants' case notes.
- Analysis of plasma and urine phosphate and creatinine concentrations from 29 to 36 weeks postmenstrual age.
- Calculation of TP/GFR, defining phosphaturia as urine phosphate > or = 1 mmol/L.
Main Results:
- In infants with phosphaturia, plasma phosphate decreased significantly with increasing postmenstrual age.
- Censored renal phosphate threshold also showed a significant decline from 29-30 to 35-36 weeks postmenstrual age.
- A statistically significant decrease (p < 0.001) was observed in both parameters.
Conclusions:
- The renal phosphate threshold appears to decrease as very low birth weight infants mature.
- This decline may be attributed to tubular reabsorption capacity increasing at a slower rate than glomerular filtration rate.
- Further research is needed to confirm the clinical implications of these findings.
Abstract:
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.