Assaying plasma prekallikrein (PPK) by a chromogenic based method: analytical considerations and reference values in healthy adults, the pregnant woman, and the neonate.

Analytical assays that measure the amidolytic activity of plasma kallikrein (PK) must overcome the problem of anti-kallikrein inhibition during the activation of PPK to PK. We resolved this problem by exposure of plasma to 50 mg/L dextran sulfate in acetone/water (35/65, vol/vol) at 0 degrees C for 15 minutes. PK activity was assayed using the chromogenic substrate, S-2302 (KABI). A centrifugal analyzer (CentrifiChem) was used to monitor the change in absorbance at 405 nm and 37 degrees C. The assay was linear to a value 3 times that found in normal human plasma. The specificity of the substrate for PK was demonstrated by using plasma deficient in PPK (Fletcher trait) diluted with normal human pooled plasma (NHPP). Day-to-day analytical variation was 2.4% for NHPP having a mean value of 85.9 mukat/L (n = 26). Using our amidolytic method, we determined PPK values in 50 healthy adults, 80 pregnant women during each trimester of gestation, and 151 neonates (cord blood specimens). The middle 95% reference interval for the adults was 54.0 - 118.8 mukat/L, for pregnant women at first trimester: 59.0 - 129.0, second trimester: 72.3 - 161.5, third trimester: 75.6 - 152.4, day of labor: 67.7 - 144.9, and 1-9 weeks postpartum: 69.4 - 151.0 mukat/L, and for the neonate: 11.5 - 44.9 mukat/L. We observed that women on oral contraceptives had significantly greater PPK values than those not on oral contraceptives. Intra-individual variation of PPK values was relatively low in non-pregnant women with the day-to-day coefficient of variation being 4.1 - 6.1% in 5 subjects monitored over a 30 day period. The high value of inter-individual/intra-individual ratio was appreciated in pregnant women in that the PPK value observed during the first trimester was a means to predict the value approaching term. The mean PPK value during the first trimester was 105.1% of the mean seen in non-pregnant women while the mean PPK value nearing term was 130.1% of that value.