Consequences for the APTT due to direct action of factor XIa on factor X, resulting in bypassing factors VIII-IX

•Contact system inhibitors identify bypass activation of factor X by factor XIa (APTT).•The bypass activity is particularly significant for low levels of FVIII or FIX.•The bypass activity is of influence on slopes of calibration lines for factor analysis.•The bypass activity is smaller with silica activator than with ellagic acid.•The bypass activity is markedly dependent on the factor XI level in factor analysis.

Back groundIt has recently been reported that factor XIa can activate factor X directly and can bypass factors VIII-IX. We evaluated the consequences for factor analysis with the one-stage APTT.MethodsAPTT was performed with the Actin FS reagent with ellagic acid as the standard. Silica, high lipid (PTT-A) or low lipid (PTT-LA) were also tested. Factor depleted and deficient plasma’s were obtained from commercial sources.ResultsThe APTT clotting times in factor XII, XI, High Molecular Weight Kininogen, factor X and factor V deficient plasma’s were all significantly longer (> 100 s) than the clotting times of factor VIII- and IX-depleted or deficient plasma’s (< 100 s). That the shorter times for factor VIII and IX deficient plasmas were due to contact activation was supported by biphasic inhibition of the clotting times with addition of Corn Trypsin Inhibitor and Trasylol. The role of factor XI and the by-passing of factor VIII/IX was shown by the use of quenching antibodies towards factor XI and VIII. Enriching factor VIII or IX depleted plasma with purified factor XI and addition of factor XIa showed a strong dependence on factor XI level. Calibration curves for factor analysis were steeper for factors FXII, HMWK, FX and FV, compared to those of both factors VIII and IX. Curves for VIII/IX were found steeper by the use of APTT-A/silica-based, 50% diluted substrate plasma and low factor XI in the substrate plasma.ConclusionsIn factors VIII and IX deficient plasmas, the APTT shows an activity which can be attributed to contact activation of factor X by factor XIa.This direct activity is lower with silica reagent compared to ellagic acid, dilution of plasma and low factor XI in substrate plasma.