Prothrombin Time (PT) is the laboratory test of choice for monitoring the
anticoagulation status of patients treated with oral anticoagulants (Coumarin,
THERAPEUTIC RANGE USING THE INR
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.
FREQUENCY OF MONITORING
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
*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.