LAUSR

Acquired factor XIII (FXIII) deficiency is a rare and life‐threatening condition that is often misdiagnosed or missed completely. A 72‐year‐old woman presented with symptoms of major unprovoked bleeding but routine coagulation screening tests and platelet count were normal. Low activated FXIII (FXIIIa) activity levels and abnormal urea clot stability led to diagnosis of acquired FXIII deficiency. A modified Bethesda inhibitor titre of 1.6 Bethesda units/ml indicated the presence of a FXIII inhibitor. Bleeding responded to a single dose of FXIII concentrate and immunosuppression with prednisolone induced remission. A subsequent relapse was treated with combined prednisolone and Rituximab resulting in a prolonged, ongoing remission. Here we analyse the mechanisms underlying this idiopathic case of acquired FXIII deficiency. Prospective analysis of patient plasma revealed minimal FXIIIa activity and antigen in presentation and relapse samples. Thrombi formed from these samples lysed rapidly and showed an absence of cross‐linked α2AP. Western blotting revealed the presence of FXIII‐B, indicating only FXIII‐A and FXIII‐A2B2 were affected. FXIII activity and antigen levels normalised on remission. Our data suggest the presence of inhibitor‐induced clearance of FXIII from plasma. As a consequence, reduced thrombus stability was evident due to defective α2AP cross‐linking, thereby explaining symptoms of excessive bleeding.