The Prothrombin Time (PT) is the laboratory test of choice for monitoring the anticoagulation status of patients treated with oral anticoagulants (Coumarin, Warfarin, Coumadin).


The activity of the thromboplastin reagent used in the Prothrombin Time (PT) test varies from lot to lot. Standardization of the Prothrombin Time (PT) with the International Normalized Ratio (INR) allows for uniform measurement of the anticoagulation status of patients on oral anticoagulants. The use of the INR has permitted development of effective recommendations for use of oral anticoagulants in a variety of clinical settings. Most published studies indicate that in most cases an INR of at least 2.0 is required for effective anticoagulation. The risk of bleeding increases with an increasing INR, and may increase dramatically above an INR of 4.5 - 5.0.


INITIATION PHASE: During the initiation phase of oral anticoagulant therapy, the patient's status should be monitored 4-5 times per week until some degree of consistency in the INR response is noted.

STABLE PHASE: Once the anticoagulant dose and INR response stabilizes the INR should be determined on an individual patient basis. But, in general, the INR should be monitored at least once each 4 weeks.

TRANSITION PHASE: A transition phase occurs whenever there are changes to medication or medical condition. A variety of drugs as well as significant dietary changes can significantly alter the response to oral anticoagulant therapy. Therefore, it is very important to more closely monitor the INR when patients in the stable phase experience changes to medication or diet.

CONCURRENT HEPARIN THERAPY: During periods in which patients are treated with both heparin and an oral anticoagulant, the effect of heparin upon the Prothrombin Time (and INR) can lead to over-estimation of the therapeutic level of oral anticoagulation. During this time, it is important to closely monitor the INR during the transition to oral anticoagulation especially if this time occurs during the early outpatient period.


  • The presence of anti-phospholipid antibodies (lupus anticoagullants) can alter the PT and give rise to INR values that can result in incorrect dosing. Contact the laboratory for recommendations if a lupus anticoagulant is suspected (409-772-3314).
  • The INR is specifically intended to improve the management of orally anticoagulated patients. Since it is not known how well the INR correlates with diagnosis or outcome in other clinical settings its use in these situations should be restricted. In most other clinical settings in which identification of defects in the extrinsic clotting cascade is important, the Prothrombin Time (PT) is likely to be the more appropriate test.

Recommended Therapeutic Range for Warfarin/Coumadin Therapy*




Treatment of venous thrombosis

Low Intensity

2.0 - 3.0

Treatment of pulmonary embolism

Prevention of systemic embolism

Tissue heart valves

Acute myocardial infarction

Atrial fibrillation

Recurrent embolism

High Intensity

2.5 - 3.5

Mechanical heart valve

Antiphospholipid antibodies+

*Hirsh J, Dalen JE, Deykin D, Poller L, Bussey H. Oral anticoagulants: mechanism of action, clinical effectiveness, and optimal therapeutic range. Fourth ACCP Consensus Conference on Antithrombotic Therapy. Chest. 1995; 108(suppl):231S-246S.

+Many investigators believe that antiphospholipid antibodies are an indication for high-intensity warfarin therapy.

Reviewed 4/5/2010 by W.Russo, MT (ASCP)