WRF - Clinical: Warfarin, Serum

Test Catalog

Test Name

Test ID: WRF    
Warfarin, Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

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.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

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.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Therapeutic concentration: 2.0-5.0 mcg/mL

Toxic concentration: > or =10.0 mcg/mL

Interpretation Provides information to assist in interpretation of the test results

Therapeutic concentration: 2.0 to 5.0 mcg/mL


Toxic concentration: > or =10.0 mcg/mL

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

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 Recommendations for in-depth reading of a clinical nature

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