Normal INR in a blood test

Each laboratory must establish its own normal range for INR in a blood test. These ranges depend on the makeup of the local population, the technologies used and the accuracy of the measurement. There may be also slight differences in the normal levels according to age, gender, race or ethnic origin, geographic region, diet, type of sample and other relevant status.

Your doctor will study the results along with your medical record, screenings, physical condition, symptoms and any other relevant information about your situation.

During pregnancy, the INR normal range may be a bit lower and values between 0.8 and 1.09 are considered to be in the normal range.

INR is the acronym of International Normalized Ratio. It appears in a blood test along with prothrombin time (PT) in the coagulation screen section.

Coagulation cascade, also known as clotting cascade, is the process activated in the body to stop a hemorrhage (a bleeding).

An important parameter measured to test the coagulation process is the prothrombin time (PT). One of the main role of the prothrombin time (PT) measurement is to adjust the anticoagulant medication dose in people under treatment for a cardiovascular disorder (venous thrombosis, myocardial infarction risk, pulmonary embolism, atrial fibrillation, etc.).

To perform the prothrombin time (PT) test it is necessary to add thromboplastin to a blood sample. However, labs can use different types of thromboplastin and for that reason, the prothrombin time tests performed with different lot numbers of reagents or different methodologies may differ by 10% to 20% from one laboratory to another.

This can be problematic when adjusting oral anticoagulant dosages based on PT test results. The dose of anticoagulant drugs to be prescribed may depend not only on the PT test results, but also on the lab that performs the blood test.

In 1983, the World Health Organization (WHO) developed a plan to standardize the PT by comparing various commercial preparations of rabbit brain thromboplastin with the more sensitive human brain tissue thromboplastin. This comparison of the thromboplastin from the animal source to the purer human source gives the International Sensitivity Index (ISI). The ISI, obtained from the manufacturers of the thromboplastin reagents, is used with the PT ratio to obtain the INR.

INR represents the PT ratio which would have been obtained for a particular patient sample as if the WHO reference thromboplastin itself (ISI=1.0) had been used in the PT determination.

INR= (Patient PT in seconds / Mean of the normal PT range for a healthy patient)^ISI

The INR (International Normalized Ratio) is used to monitor the dose of patients under anticoagulant therapy. The appropriate dose will be adjusted depending on the INR from the successive blood tests.

Recommendations for monitoring oral anticoagulant therapy (with warfarin or acenocoumarol), are usually an INR of 2.0 - 3.0 for most clients, except those with mechanical heart valves, who require a higher INR of 2.5 - 3.5.

Clients usually reach an INR of 2.0 - 3.0 in 3 - 5 days after oral anticoagulation is begun. A lower INR (1.3 - 1.9) may be the goal if the client is on low-dose warfarin to prevent thrombosis from a central venous catheter.

The INR is not used to monitor clients with liver disease. In liver disease, other clotting factors are decreased, so the INR does not detect or adjust to these changes.

The following values are considered to be normal values:

IMPORTANT: These levels are expressed in units. They are an example of a healthy man/woman of about 40 years old with no known disease and not taking any medication. The ranges can be different depending on the laboratory or on your personal circumstances.