[Clinical problems with oral anticoagulation -- 3 case reports].

Two patients with severe bleeding complications under oral anticoagulant treatment are presented, in one case caused by pharmacokinetic drug interference (phenylbutazone), in the other by genetic predisposition to bleeding induced by coumarin anticoagulants. Another patient with decreasing INR due to drug interference (rifampicin) is presented as well. The possibility of drug interferences with coumarin anticoagulants has to be anticipated, whenever the medication of an orally anticoagulated patient is changed. A founder mutation of the factor IX propeptide constitutes a genetic predisposition to bleeding in patients put on coumarins. Its presence should be excluded in any patient suffering from hemorrhagic complications after starting anticoagulation when INR values are in the target range.