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Warfarin, Serum or Plasma

CPT: 80375
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  • Coumadin®
  • Jantoven®
  • Sodium Warfarin

Expected Turnaround Time

2 - 3 days

Related Documents

For more information, please view the literature below.

Procedures for Hemostasis and Thrombosis: A Clinical Test Compendium

Specimen Requirements


Serum or plasma


2 mL

Minimum Volume

0.6 mL


Red-top tube, lavender-top (EDTA) tube, or green-top (heparin) tube. Do not use a gel-barrier tube. 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.


Transfer separated serum or plasma to a plastic transport tube.

Storage Instructions

Room temperature

Stability Requirements



Room temperature

14 days


14 days


14 days

Freeze/thaw cycles

Stable x3

Causes for Rejection

Gel-barrier tube

Test Details


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.


Liquid chromatography/tandem mass spectrometry (LC/MS-MS)

Reference Interval

1.0−10.0 μg/mL

Critical Value

Potentially toxic: >10.0 μg/mL

Additional Information

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. 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. 1401182
3. Williamson EM. Drug interactions between herbal and prescription medicines. Drug Saf. 2003; 26(15):1075-1092 (review). 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.


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
071423 Warfarin (Coumadin), Serum 4098-0 071424 Warfarin (Coumadin), Serum ug/mL 4098-0

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