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Summary Background Corn trypsin inhibitor (CTI), an inhibitor of FXIIa, is used to prevent plasma coagulation by contact activation, to specifically investigate tissue factor (TF)-initiated coagulation. Objective In the present work the specificity of CTI for factor (F) XIIa is questioned. Methods and results In the commercial available plasma coagulation assays CTI was found to double activated partial thromboplastin time (APTT) at a plasma concentration of 7.3 ± 1.5 μm CTI (assay concentration 2.4 μm). No effect was found on the prothrombin time (PT) when high TF concentrations were used. Also, with specific antibodies for FXIIa and for FXIa only APTT was found to be extended but not PT. With specific enzyme assays using chromogenic substrates CTI was shown to be a strong inhibitor of FXIIa and a competitive inhibitor of FXIa with Ki = 8.1 ± 0.3 μm, without effect on the coagulation factors FVIIa, FIXa, FXa and thrombin. In thrombin generation and coagulation (free oscillation rheometry, FOR) assays, initiated with low TF concentrations, no effect of CTI (plasma concentrations of 4.4 and 13.6 μm CTI, 25 resp. 100 mg L−1 in blood) was found with ≥ 1 pm TF. At ≤ 0.1 pm TF in the FOR whole blood assay the coagulation time (CT) concentration dependently increased while the plasma CT became longer than the observation time. Conclusion To avoid inhibition of FXIa and the thrombin feedback loop we recommend that for coagulation assays the concentration of CTI in blood should be below 20 mg L−1 (1.6 μm) and in plasma below 3 μm. |
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