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Test Code WRF Warfarin, Serum

Performing Laboratory

Mayo Medical Laboratories in Rochester

Reporting Name

Warfarin, S

Specimen Type

Serum Red

Specimen Required

Container/Tube: Red top

Specimen Volume: 3 mL

Collection Instructions: Spin down within 2 hours of draw.

Reject Due To


Mild OK; Gross OK


Mild reject; Gross reject


Mild OK; Gross OK


Serum gel tube


Specimen Stability Information

Specimen Type Temperature Time
Serum Red Refrigerated (preferred) 21 days
  Ambient  28 days
  Frozen  28 days

Specimen Minimum Volume

1.1 mL

Day(s) and Time(s) Performed

Tuesday; 11 a.m.

Specimen Retention Time

14 days

Analytic Time

2 days

Reference Values

Therapeutic concentration: 2.0-5.0 mcg/mL

Toxic concentration: ≥10.0 mcg/mL

Useful For

Monitoring patients whose prothrombin time is inconsistent with the prescribed warfarin dose, particularly when failure to comply or surreptitious drug use is suspected.

Note: This test is not useful for evaluation of the patient with prolonged bleeding time suspected of exposure to rat poisons.

Method Name

High-Performance Liquid Chromatography (HPLC)

Test Classification

This test was developed and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
WRF Warfarin, S 4098-0


Result ID Test Result Name Result LOINC Value
8760 Warfarin, S 4098-0

Clinical Information

Warfarin (Coumadin) is an anticoagulant that acts by antagonizing the action of vitamin K resulting in the same coagulation abnormalities produced by vitamin K deficiency.


Warfarin reduces the levels of prothrombin and factors VII, IX, and X, thereby prolonging the prothrombin and partial thromboplastin times.


Warfarin produces its anticoagulant effect within 36 to 72 hours of initiating therapy, and the duration of action may persist for 4 to 5 days following withdrawal of drug.


Warfarin circulates almost completely bound to albumin (>98%), and its half-life ranges from 20 to 60 hours.


Abnormal bleeding is the chief complication of overdose.


Therapeutic concentration: 2.0 to 5.0 mcg/mL


Toxic concentration: ≥10.0 mcg/mL


This test is not useful for evaluation of the patient with prolonged bleeding who are suspected of exposure to rat poisons.


Patients transfused with fresh-frozen plasma may have a disparity between their warfarin level (elevated) and a relatively normal prothrombin.

Clinical Reference

Gallus A, Jackaman J, Tillett J, et al: Safety and efficacy of warfarin started early after submassive venous thrombosis or pulmonary embolism. Lancet 1986;2:1293-1296

Method Description

Serum is extracted with petroleum ether. The extract is chromatographed and the distinctive peaks of the coumarins are identified and quantitated in the ultraviolet spectrum.(O'Reilly RA, Aggeler PM, Hoag MS, Leong L: Studies on the coumarin anticoagulant drugs: the assay of warfarin and its biologic application. Thromb Diath Haemorrh 1962;8:82-95; Fishwick FB, Taylor A: The determination of warfarin in animal relicta. Analyst 1967;92:192-195)


If not ordering electronically, complete, print, and send a Cardiovascular Test Request Form (T724) with the specimen (