Molecular Pathology Laboratory Network

On Jan. 22, 2010, the US Food and Drug Administration (FDA) updated the "dosing and administration" labeling for the anticoagulant warfarin (Coumadin).

 The FDA initially updated the drug's label in 2007 to reflect the influence of the CYP2C9 and VKORC1 genes in metabolizing the drug. Although the agency recommended doctors consider lower initial doses for patients with variations in these genes, at the time, the FDA held off on making specific dosing recommendations until additional studies were completed.

The drug's label now states: "The patient's CYP2C9 and VKORC1 genotype information, when available, can assist in selection of the starting dose." The label then instructs healthcare providers to refer to a table containing stable maintenance doses observed in multiple patients having different combinations of CYP2C9 and VKORC1 variants. "Consider these ranges in choosing the initial dose," the label recommends.

Patient genotype information can be determined with the Warfarin Sensitivity Test (Nanosphere, Inc.) offered by MPLN. This assay detects and genotypes the *2 and *3 alleles of the CYP2C9 gene and a single polymorphism (1173C>T) in the VKORC1 gene to assist physicians with prescribing the correct dosage of warfarin.

For more information about the Warfarin Sensitivity Test, contact a technical service specialist at 800-932-2943.