Comparative study of laboratory characteristics in coagulation testing for acquired factor V inhibitors

Abstract

Acquired factor V inhibitor (AFVI) is an autoantibody against factor V (FV). AFVI is a relatively rare autoimmune disease among acquired coagulation factor inhibitors and there have been limited case reports of AFVI, thus the laboratory characteristics of AFVI had been shown in some variation. In this study, we investigated four cases of AFVI in order to understand the characteristics of laboratory findings in AFVI. We compared the laboratory test characteristics with cases of vitamin K deficiency (VKD) and congenital FV deficiency (CFVD), both of which present with prolonged PT and APTT. As a result, the APTT in patients with AFVI or CFVD was more prolonged against the prolongation of PT compared to VKD. This would be due to the different sensitivity of the reagents to FV activity. On the other hand, AFVI and CFVD did not differ significantly from each other in the degree of prolongation of APTT relative to PT, but AFVI showed an inhibitor pattern in the PT- and APTT-mixing test, which could be distinguished from CFVD, which showed a coagulation factor deficient pattern. These results suggested that AFVI could be distinguished from CFVD using PT- and APTT-mixing tests. All patients with AFVI enrolled in the current study would carry a neutralizing antibody against FV, thus we could not investigated patients with clearance antibody against FV. This was a limitation of this study. However, the characteristics of AFVI could be evaluated by comparing the degree of PT and APTT prolongation according to the coagulation factor sensitivity of the assay reagents, and by considering the results of the mixing test. The certain predictions can be made before obtaining the results of FV activity or FV inhibitor titer, and it might be possible to distinguish AFVI from VKD and CFVD in combination routine laboratory tests with mixing test.