The purpose of this quasi-experiment study, which could also be classified as a prospective observational intervention study, is to assess the impact of cytochrome P450 2C9 (CYP 2C9) and vitamin K epoxide reductase complex, subunit 1 (VKORC1) testing within a primary patient care setting.

Anticoagulation therapy with warfarin is the most common mode of treatment and prophylaxis for venous and arterial thromboembolic conditions. Warfarin is metabolized in the liver by the cytochrome P450 system, the cytochrome P450 2C9 (CYP 2C9) isoenzyme specifically, and polymorphisms in the CYP 2C9 gene have been associated with changes in metabolic function of the translated isoenzyme . These polymorphisms result in reduced metabolism of warfarin as compared to subjects having the wild type gene, consequently leading to systemic accumulation of warfarin; it is theorized that this leads to higher risk of adverse events. Other allelic variations have also been linked to changes in vitamin K conservation through their effects on vitamin K epoxide reductase complex, subunit 1 (VKORC1) . The combined impact of CYP 2C9 and VKORC1 polymorphisms on warfarin's pharmacology have recently been reported.

It is hypothesized that evaluation of genomic allelic type guided warfarin dosing will reduce thromboembolic and bleeding risks associated with warfarin therapy, and that adoption of a genetic testing strategy in a primary patient care setting would improve warfarin effectiveness and patient safety, and reduce costs to health care payers.

Inclusion Criteria:

• Female and male age range of 40-75
• Patients who are in the induction phase of warfarin
• Patients receiving warfarin to prevent or treat thromboembolic conditions (e.g., post orthopedic surgery prophylaxis, deep venous thrombosis, atrial fibrillation, pulmonary embolism, heart failure)
• Patient willing to provide informed consent prior to the specimen collection procedure
• Patient whose physician is willing to order the genetic test

Exclusion Criteria:

• Age < 40 or > 75
• Previous use of warfarin within 180 days of initiating new warfarin therapy
• Hospitalized for seven or more days before first claim for warfarin
• Previous history of genetic testing for warfarin therapy
• Known hypersensitivity to warfarin
• Patient or physician refusal to participate in the study
• Patients using warfarin residing in Olmsted County, MN

The active study population consists of male and female covered lives with Medco for clients have agreed to enroll their members for ages 40-75 for new warfarin starts who match the inclusion criteria.