Factor V inhibitors: rare or not so uncommon? A multi-laboratory investigation.

Acquired deficiencies of, or inhibitors to, factor V are considered rare events. We report a series of 14 acquired factor V deficiencies, 10 of which were confirmed to have inhibitors to factor V, as identified within Australia in the past 5 years following a multi-laboratory investigation. The initial index case seen by one laboratory was followed within 4 months by a separate similar case. This prompted local contact with colleagues (n = 20) working in other haemostasis referral laboratories to identify the current case series. In total, nearly one-half of all haemostasis referral laboratories contacted had seen a case within the past 5 years. Clinical features and the apparent associated risk of bleeding complications generally varied, as did laboratory findings and the likely causal event. There were three females and 11 males. Age ranged from 44 to 95 years (median, 81 years). The level of inhibitor ranged from undetectable to over 250 Bethesda units. The probable cause leading to development of the inhibitors ranged from exposure to bovine thrombin, exposure to antibiotics, surgery and malignancy. Of additional interest was the apparent association of anti-phospholipid antibodies in many of the cases. For example, in the two similar index cases, with factor V inhibitor titres > 200 Bethesda units, high levels of anti-cardiolipin antibodies (> 70 GPL units) were also detected. Although less clear because of inhibitor interference, many of the cases also showed evident co-associated lupus anticoagulant activity. In conclusion, we report a series of factor V inhibitors recently identified within our geographic region that would represent an annual incidence of around 0.29 cases per million Australians. Although considered a rare finding, there is a high likelihood that most haemostasis referral laboratories will see a case every five or so years.