A novel ELISA for the detection of active form of plasminogen activator inhibitor-1 based on a highly specific trapping agent

Plasminogen activator inhibitor-1 (PAI-1) is a labile molecule that exists in four different forms: active, latent, cleaved and target bound form. Although there have been many methods to measure the total PAI-1, the measurement of active form of PAI-1 antigen is still challenging. Here we developed a novel ELISA to detect the active form of PAI-1 based on a highly specific PAI-1 capturing agent which binds to active PAI-1 with high affinity. We also used a highly stable PAI-1 mutant as an assay calibrator to enhance the method's reproducibility. This ELISA has the advantage of measuring both the antigen level and activity of PAI-1 at the same time. The assay had a sensitivity of 0.167 ng/ml and a working range of 0.195-25 ng/ml. The intra- and inter-assay variations were 6.7% and 11.3% respectively. The mean recovery of spiked standard was 102%. We used this strategy to measure the active PAI-1 level in plasma of healthy donors, and had an interesting observation: the PAI-1 level reduced by half after plasma storage for 6 h at room temperature. This finding represents the first observation of activity loss in plasma PAI-1 samples, and may explain large variations in PAI-1 levels (0-100 ng/ml) observed in human samples using commercial assays.