Warfarin dosage requirements: prospective clinical trial of a method for prediction from the response to a single dose

Intestinal toxicity of oral warfarin intake in rats

Though warfarin is extensively used in the prevention and treatment of thromboembolic processes in humans, adverse effects of warfarin therapy have been recognized. Intestinal hemorrhage is one of the hazards of anticoagulant therapy, but the mechanisms of warfarin toxicity are virtually unknown. In this work, the effects of 30 days oral warfarin (0.35 mg/l and 3.5 mg/l) intake on rat's gut were examined. Both doses resulted in prolongation of prothrombin time. Systemic effects of higher warfarin dose (increases in plasma AST, proteinuria, hematuria, changes in peripheral blood hematological parameters) were seen. Warfarin intake resulted in histologically evident tissue damage, leukocyte infiltration and intestinal inflammation [increases in myeloperoxidase activity, malondialdehyde content, superoxide dismutase and catalase activity, proinflammatory cytokine (IFN-γ, IL-17) concentrations in intestinal homogenates]. In contrast, suppression of gut-draining mesenteric lymph node (MLN) cell activity [proliferation responsiveness, production of IFN-γ and IL-17 to T lymphocyte mitogen Concanavalin A stimulation] was noted. Inhibition of regulatory cytokine IL-10 production by MLN cells, suggests commitment of MLN to the suppression of all inflammatory activities and creation of the microenvironment which is non-permissive for induction of potentially harmful immune response. These novel findings indicate the need of staying alert for (adverse) effects of warfarin therapy.

The initial phase of oral anticoagulation with warfarin in outpatients with deep venous thrombosis

Patients with deep venous thrombosis (DVT) treated out of hospital usually start warfarin with the recommended 5 mg loading dose and have their International Normalized Ratio (INR) test performed every 2-3 days. Thus, achievement of the therapeutic range may be more difficult than for inpatients, possibly resulting in extended duration of low molecular weight heparin (LMWH) treatment. We retrospectively examined the charts of 55 DVT outpatients (mean age, 61.4 years; 30 males) to assess the actual duration of LMWH treatment and to identify predictors of a slow achievement of the INR range. Thirty patients (54.4%) reached the therapeutic INR range and stopped LMWH within 7 days, and 25 patients (45.6%) had to continue for an average of 10.5 days. The latter group was significantly younger than the former (57 and 65 years, respectively; P = 0.039). Patients younger than 60 years old had an odds ratio for an extended treatment of 4.92 (P = 0.0057). Algorithms with different loading doses of warfarin according to age should be proposed for outpatient treatment of DVT.

Long-term anticoagulation. Indications and management

Each year half a million persons in the United States receive long-term anticoagulant therapy to prevent venous and arterial thromboembolism. Unfortunately, the relative benefits and risks of anticoagulant therapy have not been adequately quantified for many thromboembolic disorders, and the decisions as to whether, for how long, and how intensely to administer anticoagulation are often complex and controversial. Several expert panels have published recommendations for anticoagulant therapy for different thromboembolic disorders; the primary area of disagreement among these panels concerns the optimal intensity of anticoagulation. Recent research and analytic reviews have helped to clarify both the risk factors for and the appropriate diagnostic evaluation of anticoagulant-induced hemorrhage. Clinicians must be aware of the nonhemorrhagic complications of anticoagulant therapy, particularly during pregnancy. The administration of anticoagulants is difficult both in relation to dosing and long-term monitoring. Knowledge of the pharmacology of the anticoagulants, an organized approach to ongoing monitoring, and thorough patient education may facilitate the safe and effective use of these drugs.