[Resistance to vitamin K antagonists. 6 cases]

Haemorrhage is the most frequent complication of oral anticoagulant therapy (OAT) and a resistance to these drugs is rarely reported. The following classification of OAT resistance is proposed: primary or secondary resistance according to the delay of onset (at the initiation of therapy or later); selective or generalized according to the number of drugs involved (only one or several); absolute or relative as judged by the prolongation of the prothrombin time (absent or moderate). Six cases are reported and the mechanisms of resistance are discussed: no intake, variations in the vitamin K availability (diet, intestinal absorption and synthesis of vitamin K), variations in the pharmacokinetics of oral anticoagulants (drug interactions, abnormal hepatic metabolism) and variations in the receptor affinity (hereditary resistance). Resistance is often overcome by progressive increase of the doses of oral coagulant, or by changing drugs, to warfarin or coumadin (long acting drugs).