Criteria and Search for Specific Assays for Active Plasminogen Activator Inhibitor 1 (PAI-1) in Plasma

BACKGROUND: Proficiency testing of enzymatic methods for plasminogen activator inhibitor 1 (PAI-1) showed a systematic variability indicating the aspecificity of some methods. METHODS: To define and detect specificity of enzymatic methods for PAI-1, experts of the ISTH/SSC subcommittee defined criteria and test samples to check the criteria. 16 samples were prepared to test (a) specificity in depleted plasma, (b) interference by added PAI-2 or PAI-3, (c) interference by added tissue-type plasminogen activator (t-PA), (d) performance in dose response. To exercise the test procedure participants were recruited via the subcommittee, literature and company data. Coded samples were distributed to participants. The NIBSC standard for PAI-1 activity was included for the normalisation of results. Adherence to the predetermined criteria was judged blind in the ISTH/SSC subcommittee meeting. RESULTS: In total 17 laboratories with 15 different assay methods participated in the study. Methods were based on four detection principles. 8 methods failed to detect sufficiently low activity in depleted plasma and were sensitive to added PAI-2. 10 methods were sensitive to interference by endogenous t-PA as revealed by additions of t-PA. Not all methods could adequately measure in acidified plasma. Two methods were fulfilling all the criteria CONCLUSIONS: Most methods were not specific for PAI-1 (enz. procedure) in acidified plasma. Definition of criteria and test methods by experts of the ISTH/SSC proved a valuable concept, according to the exercise undertaken.