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Unit Code 8760:
Warfarin, Plasma

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Useful For

Monitoring patients whose PT 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

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 (PT) and partial thromboplastin

times (PTT).

 

Warfarin produces its anticoagulant effect within 36-72 hours of

initiating therapy, and the duration of action may persist for 4-5

days following withdrawal of drug.

 

Warfarin circulates in plasma almost completely bound to albumin

(>98%), and its plasma half-life ranges from 20-60 hours.

 

Abnormal bleeding is the chief complication of overdose.

Reference Values

Therapeutic concentration:  2.0-5.0 ug/mL

Toxic concentration: > or =10.0 ug/mL

Interpretation

Therapeutic concentration:  2.0-5.0 ug/mL

 

Toxic concentration:  > or =10.0 ug/mL

 

Cautions

Patients transfused with fresh frozen plasma may have a disparity

between their warfarin level (elevated) and a relatively normal PT.

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

Key