Measuring plasminogen activator inhibitor activity in plasma by two enzymatic assays

We compared two methods that measure plasminogen activator inhibitor (PAI) activity in plasma based on the ability of PAI to inhibit tissue plasminoge n activator (tPA) or urokinase (uPA) in order to determine which method mos t accurately measures plasma PAI activity after stressors, like hemorrhage. Plasma PAI activity was significantly elevated after hemorrhage in both as says. Using standard curves derived from rhPAI-l, we found that the tPA-PAI assay was more sensitive than the uPA-PAI assay. However, we measured a 10 -fold difference in PAI activity as measured between assays, suggesting tha t some endogenous plasma constituents (tPA, uPA, plasminogen or plasmin) ma y interfere with the accurate determination of PAI activity. Increasing the amount of plasma in each assay led to a progressive increase in PAI activi ty. However, removing either tPA or plasminogen from the tPA-PAI assay unma sked the presence of some endogenous tPA and plasminogen. Furthermore, incr easing plasma volume in either assay increases measured plasma tPA, but not uPA. Finally, plasma tPA is elevated after hemorrhage, whereas plasma uPA is not. These results suggest that endogenous tPA and plasminogen may inter fere with the measurement of plasma PAI activity in the tPA-PAI assay after hemorrhage or other stresses. The uPA-PAI assay does not have this confoun ding problem because endogenous uPA does not interfere with the assay, nor does it rise during hemorrhage. (C) 2000 Elsevier Science B.V. All rights r eserved.