The values of the MNPT and ISI parameters come from clinical
calibration studies.
The pharmacological activity of vitamin K-antagonist
oral anticoagulants can be modified by other drugs;
therefore, you should only take the medicines
prescribed by your physician.
Certain liver diseases, thyroid disorders and other
diseases or conditions, as well as nutritional
supplements, medicinal herbs or changes in diets can
affect the therapeutic activity of oral anticoagulants
and INR results.
04
1.3 ORAL ANTICOAGULANT THERAPY
Oral anticoagulant therapy is given to patients to prevent
thromboembolic events such as venous thrombosis and
pulmonary embolism or those linked to atrial fibrillation or
artificial heart valves.
The treatment entails the need to monitor and adjust the
doses periodically for each patient based on a blood test.
Depending on the reason for oral anticoagulant therapy a
therapeutic range is defined for each patient meaning the
value of the test should lie within that range.
Prothrombin Time and INR
The activity of oral anticoagulants is monitored by measuring
the prothrombin time (PT) in seconds, which is the time it
takes for a fibrin clot to form. The thromboplastin is used as a
reagent to calculate the prothrombin time (PT). Depending on
the nature of this reagent and the equipment used variations
of the PT results are to be expected.
Therefore, in 1977, the World Health Organization (WHO)
recommended a system standardisation method. Thus,
prothrombin time values are converted into INR values,
International Normalized Ratio, using the following equation:
Where PT is the prothrombin time obtained in the test, MNPT
is the mean normal prothrombin time and ISI the international
sensitivity index corresponding to the thromboplastin.
INR =
PT
ISI
MNPT
( )
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