Warfarin, Serum or Plasma

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.^1,2

Assay measures sodium warfarin only.

This test was developed and its performance characteristics determined by LabCorp. It has not been cleared or approved by the Food and Drug Administration.

Warfarin is intermediate-acting; a peak effect is achieved in 36 to 72 hours and the anticoagulant effect persists for two to five days. The drug is rapidly and completely absorbed from the gastrointestinal tract. The delay in achieving the peak effect results from the long half-lives of clotting factors II, IX, and X, whose levels must decline before maximal anticoagulation is reached. Warfarin is highly lipophilic and is 99% bound to albumin. Some tissue binding occurs, particularly in the liver; however, only the unbound drug is active and available for hydroxylation in the liver. Therefore, co-administered agents which either increase the free fraction (eg, salicylates) or activate liver enzymes (eg, St John's wort) need to be monitored cautiously.³

1. Montamat SC, Cusack BJ, Vestal RE. Management of drug therapy in the elderly. N Engl J Med. 1989 Aug 3; 321(5):303-309. PubMed 2664519

2. James AH, Britt RP, Raskino CL, Thompson SG. Factors affecting the maintenance dose of warfarin. J Clin Pathol. 1992 Aug; 45(8):704-706. PubMed 1401182

3. Williamson EM. Drug interactions between herbal and prescription medicines. Drug Saf. 2003; 26(15):1075-1092 (review). PubMed 14640772

AMA, Division of Drugs and Toxicology. Drug Evaluations Subscription. Chicago, Ill: American Medical Association, Spring 1992.

Mosby's Drug Consult. St Louis, Mo: Elsevier Mosby; 2006.