Warfarin is used for chronic oral anticoagulation in a variety of clinical settings. Management of warfarin therapy is usually by following the prothrombin time, rather than by measuring serum drug concentrations. Warfarin is subject to a bewildering number and variety of drug interactions, producing increased or decreased clinical effect of itself or other drugs. Many of these effects are due to changes in protein binding or hepatic metabolism. Reductions in dosage may be indicated for aging subjects treated for venous thromboembolic or coronary arterial disease, but not in those with peripheral vascular disease, deep vein thrombosis, or valvular heart disease.
The use of gel-barrier tubes is not recommended due to slow absorption of the drug by the gel. Depending on the specimen volume and storage time, the decrease in drug level due to absorption may be clinically significant.
Spin and separate
Ambient (preferred) - 14 days
Refrigerated - 14 days
Frozen - 14 days
Liquid chromatography tandem mass spectrometry (LC-MS/MS)