Evaluation of fetal lung maturity by measurement of the procoagulant activity in amniotic fluid.

The procoagulant activity of amniotic fluid (AF) increases with advancing gestational age. AF samples (N = 276) were measured by a modified recalcification time method. The results were expressed as equivalents of Russell's viper venom (RVV) which acts in a similar way as AF by way of direct activation of coagulation factor X. After week 35 of pregnancy RVV values reached or passed 0.09 micrograms/ml. This value represents a threshold indicating fetal lung maturity. In 124 cases postnatal lung maturity was related to RVV equivalents. The predictive value of a mature result was 99.1%, that of an immature one 52.2%. When comparing RVV values with the lung status, surface tension measurements, and L/S ratio a high correlation was found. L/S ratio and procoagulant activity had a correlation coefficient of r = 0.60. Betamethasone seemed to produce the expected changes in the results of RVV assays and surface tension measurements.