Abstract: The present invention relates to medicine, hematology, biotechnology and fundamental biological research; it can be used for diagnostic and research purposes when studying blood coagulation. One of the aspects of the invention is a polypeptide for selective inhibition of contact activation factor XIIa in a sample of blood or plasma, which comprises the sequence of infestin-4 MutB (SEQ ID NO: 1), where this sequence can have modifications outside the area of the inhibitory loop. Another aspect of the invention is a method of studying coagulation in the test sample, which includes bringing the sample into contact with this polypeptide, bringing a coagulation activator into contact with the sample, and measuring the coagulation parameters. Yet another aspect of the invention is a method of blood anticoagulation by this polypeptide for collection purposes and in order to increase the storage time of the sample.

Abstract: The present invention relates to medicine, and specifically to hematology, and can be used in particular for diagnostic and research purposes when determining coagulation characteristics of blood and its components, and also in biotechnology and fundamental biological research. One of the aspects of the present invention is a polypeptide for inhibition of contact activation in a sample of blood or its product to be tested, which comprises the sequence of an infestin-4 mutant MutB (SEQ ID NO: 1) and which substantially corresponds thereto, where said sequence can have modifications outside the area of the inhibitory loop which significantly maintain the activity of the said polypeptide, which acts as a high-selectivity fXIIa inhibitor, the selectivity of which is higher than that of native infestin-4 and Mut15, or the activity of which is higher than that of native infestin-4 and Mut15.