Method for excluding factor xia inhibitors in patient samples

A method for excluding an anticoagulant from the group of a factor XIa inhibitors in a patient sample, the method comprising the steps of:

• a) preparing a first reaction mixture containing the patient sample and a second reaction mixture containing a normal plasma sample by mixing each of the patient sample and the plasma sample with
• i) a factor XI-deficient plasma, and
• ii) at least one neutralizing substance which neutralizes an anticoagulant other than an anticoagulant from the group of factor XIa inhibitors, and
• iii) a coagulation activator of the intrinsic blood coagulation system;
• b) measuring a first coagulation time in the first reaction mixture (APTT XISample) and a second coagulation time in the second reaction mixture (APTT XINormal);
• c) forming a quotient APTT XISample/APTT XINormal from the first and the second coagulation times;
• d) comparing the quotient with a predetermined reference value;
• and
• e) excluding anticoagulants from the group of factor XIa inhibitors in the patient sample if the quotient APTT XISample/APTT XINormal is below the predetermined reference value.