Evaluation of proximal tubular function in preterm infants by urinary α1-microglobulin

α₁-Microglobulin is a low molecular weight protein that is relatively stable in urine of low pH. There have been few reports on urinary α₁-microglobulin (U-A₁M) excretion in preterm infants. This study was designed to establish the ranges for U-A₁M in clinically stable preterm infants and to investigate changes observed in sick preterm infants. We measured U-A₁M and urinary β₂-microglobulin (U-B₂M) levels at 1, 4, 7, 14, 28 and 90 days after birth in stable preterm infants (Group 1) and sick preterm infants who were depressed at birth and required immediate resuscitation (Group 2). In Group 1 infants, both parameters were high during the first 28 days and appeared to decline thereafter. U-A₁M in Group 2 infants was only significantly increased compared with Group 1 on day 1, as was U-B₂M. On each day of the study, U-A₁M had significant positive correlations with U-B₂M for all the infants studied. The changes of the two parameters observed in Group 1 probably reflect postnatal evolution of proximal tubular function in stable preterm infants. A comparison of groups 1 and 2 shows a high prevalence of acute tubular injury at birth in sick infants and also suggests that U-A₁M as well as U-B₂M may be a sensitive index for detecting acute tubular damage and for following its course in preterm infants.