Protein analysis in amniotic fluid and fetal urine for the assessment of fetal renal function and dysfunction.

Micromolecular proteins (molecular weight less than 68 kD) in amniotic fluid are assumed to be derived largely from fetal urinary excretion and therefore may reflect fetal kidney function and maturation. Microprotein concentrations in amniotic fluid, neonatal urine and urine of fetuses with bilateral urinary tract dilation were analyzed using microgradient gel electrophoresis to separate proteins according to their molecular size. Alpha-1-microglobulin and beta 2-microglobulin were assayed as singular micromolecular marker proteins. Microproteins in amniotic fluid decreased progressively with advancing gestation during the 3rd trimester. Micromolecular protein concentrations in the first postnatal urine of healthy infants born prematurely or at term were similar to those in amniotic fluid of corresponding fetal age and yielded an identical developmental pattern. A strong correlation existed between the microprotein concentrations in amniotic fluid and fetal urine. It is concluded that fetal urinary production is the main determinant for the microprotein content of amniotic fluid and that a major fetal pathway exists for the intrauterine metabolism of these proteins. The 3rd trimester decrease in amniotic fluid seems to be dependent on the increasing reabsorption capacity of proximal tubular cells representing morphological and functional kidney maturation. The exact diagnostic value of microprotein analysis for the assessment of disturbed fetal kidney function has still to be defined by further investigation.