Renal Injury in Sick Newborn Infants: A Prospective Evaluation Using Urinary ₂-Microglobulin Concentrations

¹ From the Edward Mallinckrodt Department of Pediatrics, Washington University School of Medicine, Divisions of Newborn Medicine and Nephrology, Children's Hospital, St Louis

Urinary concentrations of ₂-microglobulin and creatinine were measured serially in 140 sick infants, of whom 109 were asphyxiated, and in 35 healthy preterm and term infants. First voided urines and samples from days 3 and 7 postpartum were studied. Urinary ₂-microglobulin concentrations in healthy infants averaged 1.34 ± 1.34 mg/L (mean ± SD) in first voided specimens and 1.32 ± 0.98 mg/L in day 3 samples; the calculated upper limit of normal (95% confidence limit) was 4.00 mg/L. Elevated values (those exceeding the 95% confidence limit) occurred most often in the sick asphyxiated patients (56%); the first voided sample value in these patients was 10.0 ± 10.4 mg/L. The equivalent value in the sick nonasphyxiated infants was 8.32 ± 7.27 mg/L. Values were significantly and persistently elevated in the sick infants on days 3 and 7. Factoring ₂-microglobulin levels by urinary creatinine concentration did not affect the significance of the findings. The increased urinary ₂-microglobulin levels were not (1) related to gestational age; low ₂-microglobulin values occurred at all gestational ages for both healthy and sick infants; (2) a consequence of urine flow rate; urinary ₂-microglobulin did not correlate with urinary creatinine concentration or with urine to plasma creatinine ratio; and (3) a consequence of increased production of ₂-microglobulin; urinary and serum ₂-microglobulin values did not correlate (r = .03). Thus, we propose that the elevated levels of urinary ₂-microglobulin in the sick infants were the consequence of tubular injury. This was associated with hematuria but not with a high incidence of azotemia or oliguria. In the most premature infants (<32 weeks), elevated urinary ₂-microglobulin concentrations were associated with significantly increased urinary concentrations of sodium and potassium. These data suggest a higher prevalence of acute tubular injury in sick newborn infants than has been reported in previous studies in which more traditional indices of renal injury were used.

Key Words: proximal tubule • urinary ₂-microglobulin • kidney • asphyxia • newborn

Submitted on April 20, 1987
Accepted on June 16, 1987

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