INR (international normalized ratio) is a ratio that helps physicians monitor patients taking anticoagulant medications. The INR is a method of expressing the results of a prothrombin time (PT) blood test. It is based on an international standard that automatically corrects for variations between labs. Thus, using the INR, the PT measurement from one lab can be compared to a PT measurement from any other lab in the world, even if they use different methods to measure PT. The resulting measurement is often referred to as the INR/PT.
The INR/PT was developed specifically for patients who are taking warfarin, an anticoagulant that inhibits the formation of blood clots. Warfarin is prescribed to patients who are at elevated risk of a blood clot causing a heart attack or stroke. The INR/PT test is administered routinely to monitor the blood level of the medication, which must kept within a very narrow range to be effective while lessening risk of uncontrolled bleeding. The INR/PT test may not be effective in patients with liver disease or patients suffering from antiphospholipid syndrome. Similarly, patients who are being given heparin, another anticoagulant given intravenously during surgery or at the initiation of anticoagulation therapy, may not benefit from the test since heparin does not prolong the PT.
Aside from some changes in certain medications that may be ordered by one’s physician, no special preparations are necessary for this test. A healthy person will have an INR of 1.0. However, patients taking anticoagulants to reduce the risk of blood clots may be advised to maintain an INR between 2.0 and 3.5. A higher number indicated greater anti-coagulation. Conditions that may warrant a higher-than-normal INR include atrial fibrillation, artificial heart valves, and thrombophlebitis (e.g., deep vein thrombosis, superficial vein thrombosis). Superficial vein thrombosis is usually not treated with warfarin.
About INR/PT tests
A prothrombin time (PT) test is a type of blood test that measures how long it takes for a blood clot to form in a blood sample after the blood has been exposed to a specific reagent, or clotting factor. This test is frequently used to monitor patients who are on anticoagulant therapy.
However, due to differences in the reagents used, as well as differences in lab equipment, PT test results may vary widely from lab to lab. Some reagents cause blood to clot more quickly than others, while certain equipment may speed or slow clotting time. To correct this, in 1983 the World Health Organization developed the INR, or international normalized ratio. This ratio allows different labs to standardize their PT test results. Thus, PT test results from one lab can be compared to PT test results from any other lab in the world.
The INR is figured by dividing the raw patient PT score by a standard control PT score, multiplied by a figure known as the ISI (international sensitivity index).
INR = [Patient PT / Control PT] ISI
The ISI was derived by using an international reagent, although each lab will calculate a different ISI based on their specific equipment and reagent combination. Similarly, the control PT is unique to each lab – it is that lab’s mean normal PT, based on a large sample of blood tests that are handled identically to the typical patient test.
Using the INR/PT, physicians are able to more accurately monitor and compare the blood clotting time of patients on anticoagulant medication, specifically warfarin therapy. The INR/PT does not apply to patients taking heparin, another anticoagulant, because heparin does not slow the PT. Nor is it applicable to patients with liver disease or antiphospholipid syndrome. The INR/PT allows patients on anticoagulant therapy to travel from country to country or state to state and still monitor their therapy.
INR/PT takes only a few minutes and is relatively inexpensive, making it popular both for diagnosing clotting disorders and monitoring the progress of patients taking anticoagulants (medications that prevent the formation of potentially dangerous blood clots). The test can be performed by a physician, nurse or certified technician in a hospital, physician’s office or clinical laboratory.
Patients taking anticoagulants will need to undergo INR/PT testing regularly to monitor their medication dosage. These tests usually require the patient to visit a hospital, physician’s office or clinical laboratory monthly or weekly.
Before the INR/PT test
No changes in diet or activity are necessary before an INR/PT test. However, a medical history will be updated before the test to determine if any of the patient’s medications might interfere with the accuracy of the test or cause the INR to be too high (blood takes too long to clot) or too low (blood is more prone to clotting). Substances that may affect the INR/PT include:
Antibiotics. Medications that harm or kill microorganisms and are commonly used to treat infections. These medications may cause a deficiency in vitamin K, which is used as part of the clotting pathway.
NSAIDs (nonsteroidal anti-inflammatory drugs). Anti-inflammatory drugs like NSAIDs prolong bleeding by inhibiting the function of platelets and may directly alter the INR/PT measurement.
Aspirin. While typically used as a pain-reliever (analgesic), aspirin is also an antiplatelet agent that will influence the risk of bleeding.
Progestin/estrogen combinations. This combination of hormones is often used in oral contraceptives (“The Pill”) and hormone replacement therapies.
Vitamin K deficiency. Whether due to poor nutrition or prolonged use of antibiotics, vitamin K deficiency may prolong PT by decreasing the blood’s ability to clot.
Understanding INR/PT test results
A prothrombin time (PT) of 10 to 20 seconds is considered normal, indicating normal blood clotting. However, this raw PT result is rarely used in tracking blood coagulation over the long term. PT test results are adjusted using an international normalized ratio (INR), a mathematic ratio that adjusts for differences in the way labs conduct PT tests.
A healthy person would have an INR of 1.0. A higher INR/PT indicates that it takes blood longer to clot, while a lower number indicates that blood clots more quickly. Patients with atrial fibrillation are often told to maintain an INR of 2.0 to 3.0. Patients with artificial heart valves may be advised to maintain an INR of 2.5 to 3.5. Doing so reduces the risk of potentially fatal events such as a blood clot from the heart or from an artificial valve traveling to the brain (causing a stroke) to the lungs (causing a pulmonary embolism), to the legs (causing impaired limb circulation), to the kidneys and other areas of the body.
Although each person’s case is different, abnormal or high INR readings may cause a physician to adjust the dosage or dose-schedule of warfarin. For instance, an INR between 3.5 and 5 may result in a recommendation to skip a dose of warfarin or reduction of the maintenance dose. At higher elevations, patients may be advised to skip warfarin and be given vitamin K. Vitamin K is an essential part of the coagulation cascade. Its administration has been shown to reverse the effect of excess anticoagulation faster than suspending warfarin alone. An INR greater than 20 should generally be treated aggressively to reduce the risk of hemorrhage.
A longer than normal international normalized ratio (INR) or prothrombin time (PT) in patients not taking anticoagulants means that the patient’s blood is taking too long to form a clot. Abnormal INR/PT results are frequently seen in patients with liver disease. Additional testing will usually be ordered to determine the specific cause.
Other blood clotting tests
In addition to the prothrombin time, there are other coagulation tests that measure the blood’s ability to clot. These tests include:
Prothrombin consumption time (PCT). Detects deficiencies in blood platelets and other factors necessary for coagulation. This test measures the time it takes for a patient to generate thromboplastin.
Activated whole blood clotting time. This test measures how long it takes for a blood clot to form in a sample of freshly drawn, whole blood after an activating agent is used. It has largely replaced the activated partial thromboplastin time test (see below) in the monitoring of heparin therapy.
Activated partial thromboplastin time (APTT). Measures the time it takes for a sample of blood to clot. It is known as the “partial thromboplastin” test because of the absence of certain factors along the coagulation cascade. The test is commonly used to monitor the progress of patients who are taking certain anticoagulants, including heparin, which cannot be measured by the INR/PT test.
Thrombin time. This test measures the conversion of fibrinogen to fibrin, which is the final step in the clotting cascade. This test may be elevated if the patient has been given anticoagulant medications or if certain diseases (e.g., multiple myeloma) are present.
Coagulation factors screening tests. Used to identify both congenital (present since birth) and acquired deficiencies in clotting factors. Some of these factors may be altered in female patients taking oral contraceptives.
Fibrinogen test. Used to diagnose blood disorders in which there is either too much or too little fibrinogen – a key factor in blood clotting.
Bleeding time. A test in which three small scratches are made on the patient’s skin and then the time it takes for the bleeding to stop is recorded. It is used to assess the integrity of platelet function.
Questions for your doctor
Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions related to INR/PT tests:
Am I putting myself at any sort of risk by taking the INR/PT tests?
Are there alternate tests available that might be more appropriate or provide more information?
What type of results would you expect to see from this test? What type of results would you consider abnormal?
Do I have any medical conditions that might affect the test results?
Do I need to make any advanced preparations in order to take this test? Should I discontinue the use of any medications?
Will I need to take any additional tests in order for you to better understand my condition?
How will I find out the results of the test?
Who can I talk to if I have questions about the test results?
I previously had this type of test performed at a different lab, are the results comparable?
Do you expect to adjust my anticoagulant medication dosage after receiving the results of the test?
