Relationships between glomerular filtration rate and kidney volume in low-birth-weight neonates
Background: Low birth weight (LBW), defined as birth weight below 2,500 g, is an important risk factor for the development of hypertension and renal disease in adult life. LBW is associated with a reduced nephron number, which results in hyperfiltration. The objective of this study was to compare the glomerular filtration rates (GFRs) of LBW and normal-birth-weight (NBW) term infants relative to their kidney volumes. Methods: Term infants (born after 37 weeks of gestation) who had been admitted to Townsville Hospital's neonatal unit were recruited for this study. Serum cystatin C was used to calculate GFR. A kidney ultrasound was used to measure renal volume. All assessments were performed during the first week of life. Results: Data from 39 infants (17 male, 22 female; 13 LBW, 26 NBW) were analyzed. There were no significant differences in the median cystatin C (1.36 mg/L, inter quartile range [IQR] = 1.12 - 1.41, vs. 1.17 mg/L, IQR = 1.10 - 1.39; p = 0.39) and gestational age. There was no significant difference in the median GFR (53.0 ml/min per 1.73 m2, IQR = 50.8-66.9, vs. 63.2 ml/min per m2, IQR = 51.8-69.5; p = 0.39) between LBW and NBW infants, but LBW infants had smaller total renal volume compared with NBW infants (18.0 ± 4.7 mL vs. 24.4 ± 6.2 mL; p = 0.002). Conclusion: Within 6 days, LBW infants achieved a similar GFR to NBW infants, despite 25% smaller kidney volumes. Thus, the single-nephron glomerular filtration rate must be increased in LBW infants. Prior to this study, it was unclear when hyperfiltration begins, but our results demonstrate that hyperfiltration begins in early life.