Grossniklaus D. Testing of VKORC1 and CYP2C9 alleles to guide warfarin dosing: Test Category: Pharmacogenomic (Treatment). PLOS Currents Evidence on Genomic Tests. 2010 Sep 14 . Edition 1. doi: 10.1371/currents.RRN1155.
Warfarin is an oral anticoagulant that is widely prescribed to prevent thromboembolic events in persons at increased risk. The optimal dose is difficult to establish because it can vary 10-fold among individuals due to clinical and demographic factors. Testing for variants of the vitamin K epoxide reductase complex 1 (VKORC1) and cytochrome P450 2C9 (CYP2C9) genes has been proposed for use in guiding the initial dose of warfarin, thus achieving optimal dosing more quickly and with lower risk of bleeding.
Pharmacogenetic testing to guide warfarin dose selection for individuals at risk of a thromboembolic event with the goal of shortening the time required to achieve a stable, effective dose and minimizing the risk of adverse effects. 
Analysis of multiple single nucleotide polymorphisms (SNPs) in VKORC1 and CYP2C9 .
Public Health Importance
Over 31 million warfarin prescriptions were dispensed in 2004. Bleeding from warfarin use is a common adverse event and can cause substantial morbidity and mortality. 
Published Reviews, Recommendations and Guidelines
Systematic evidence reviews
Recommendations by independent group*
Guidelines by professional group
2008 American College of Chest Physicians guideline statement: “We suggest against pharmacogeneitc-based dosing until randomized data indicate that it is beneficial.” 
Some additional guidelines and recommendations can be found at Centers for Medicare & Medicaid Services Decision Memo for Pharmacogenomic Testing for Warfarin Response (CAG-00400N)(dated 8/3/2009):
* independent groups include the US Preventive Services Task Force (USPSTF) and Evaluation of GenomicApplications in Practice and Prevention (EGAPP) Working Group.
Analytic Validity : Test accuracy and reliability in identifying alleles at multiple SNPs (analytic sensitivity and specificity).
Clinical Validity : Test accuracy and reliability in predicting appropriate warfarin dose (predictive value).
Clinical Utility : Net benefit of test in improving health outcomes.
The ACCE review reported controlled trials that evaluate the net effect of testing for CYP2C9 and VKORC1 alleles on health outcomes in persons treated with warfarin have not been conducted. 
Recent additions to the literature:
The Center for Medicare and Medicaid Services (CMS) does not generally reimburse costs, but it does provide coverage for individuals enrolled in appropriate clinical trials designed to examine clinical utility of genetic testing for Warfarin dosing. See CMS decision memo.