Serum plasminogen and lung surfactant in the respiratory distress syndrome.

Plasminogen, total protein, and surface-active material were measured in amniotic fluid in 112 pregnancies at 11-42 weeks' gestation. In 65 of these pregnancies, cord blood was also analyzed for serum plasminogen and total protein. Plasminogen was detected in 25 of 114 amniotic fluid samples, and 23 came from pregnancies of less than 37 weeks' gestation. Plasminogen was found in 15 of 32 amniotic fluid samples from pregnancies with complications, but only in 10 of 80 "uncomplicated" pregnancies. The mean cord serum plasminogen was relatively constant in births or abortuses of 17 to 30 weeks' gestation, but was present in increasing amounts in births of gestational ages from 30 to 40 weeks. The concentration of plasminogen in cord serum was directly related to the cord total protein (r = 0.7513, P less than 0.001). The cord plasminogen concentration was significantly higher in infants with a positive foam stability test (5.6 +/- 0.3 mg/100 ml) than in the combined group of infants with negative and intermediate tests (4.3 +/- 0.16, P less than 0.005). However, infants with a positive foam stability also had a significantly greater gestational age than infants with a negative or intermediate foam stability test. With one exception, infants with a low cord plasminogen (below 4 mg/100 ml) developed respiratory distress syndrome (RDS) only if amniotic fluid surfactant was low. The data suggest that low levels of serum plasminogen are correlated with severe lung disease only in the presence of surfactant deficiency.