A Circulating Anticoagulant Against Factor V

A 65‐year‐old man who developed a potent circulating anticoagulant directed against factor V is described. After prostatectomy he went into a septic shock and received treatment with cephalothin. After 12 days' treatment he developed severe bleedings and signs of drug allergy. Coagulation studies showed a factor V level of 0% and a circulating anticoagulant against factor V in high titre. The anticoagulant belonged to the IgG class. After treatment with cyclophosphamide in large doses and withdrawal of cephalothin the bleedings stopped and the anticoagulant successively decreased, and after 5 weeks he had a normal factor V level. In this case the anticoagulant probably developed as a consequence of the cephalothin therapy.