The INR is a method to standardize results, allowing effective comparison of PT results from different laboratories.
International Normalized Ratio [INR] =
= ((observed PTR)^(ISI))
where:
• observed PTR = observed prothrombin time ratio = (patient PT) / (control PT)
• ISI = International Sensitivity Index = value determined for each manufacturer's lot of thromboplastin reagent by comparison to an international WHO reference preparation of thromboplastin, which has an ISI of 1.0 by definition. Ideally the thromboplastin used should have an ISI value in range 2.2 to 2.6.
Therapeutic use:
• INR in range 2 to 3: range for less intensive anticoagulation with coumarins
• INR in range 3 to 4.5: range for conventional anticoagulation with coumarins
|
INR & Oral Anticoagulation
|
Minimal Effective
|
Recommended
|
|
deep vein thrombosis, prevention
|
1.5 - 2.5
|
2.0 - 3.0
|
|
deep vein thrombosis, treatment
|
2.0 - 2.3
|
2.0 - 3.0
|
|
acute MI, prevention of stroke
|
2.0
|
2.0 - 3.0
|
|
acute MI, prevention of recurrence
|
2.7 - 4.5
|
3.0 - 4.5
|
|
acute MI, reduction of mortality
|
2.7 - 4.5
|
3.0 - 4.5
|
|
peripheral arterial disease
|
2.6 - 4.5
|
|
|
atrial fibrillation, prevention of emboli
|
1.5 - 2.5
|
2.0 - 3.0
|
|
cardiac replacement valve - tissue
|
2.0 - 2.3
|
2.0 - 3.0
|
|
cardiac replacement valve - mechanical
|
1.9 - 3.6
|
3.0 - 4.5
|
Source: Hirsh. N Eng J Med (1991) page 1870
Limitations:
• Minor differences in thromboplastin responsiveness and ISI assignment can affect the calculated INR.
• It should not be used for patients not receiving oral anticoagulants.
• It should not be used on patients only recently started on anticoagulation therapy.
• It should not be used in patient with severe liver disease sufficient to depress functional coagulation factor production.
• It should not be used to monitor patients with known clotting factor deficiencies affecting the PT.
