INR (international normalized ratio) testing measures the speed at which blood clots. It is commonly used to measure the clotting time of patients taking blood-thinning medications, such as warfarin (Coumadin). The results of this test are given as a ratio. INR testing is performed to evaluate the patient's blood-clotting process to make sure it is in the normal range, and that the medication in question is preventing serious blood clots without causing increased risk of dangerous bleeding.
Also known as a prothrombin time (PT) test, named after a protein produced by the liver that assists in the blood-clotting process, this test measures how long the clotting process takes and how efficiently it occurs. There are a number of steps to the clotting process; all involve a number of chemicals known as clotting factors. An important step in the process is the one in which prothrombin changes to another protein called thrombin. Measuring the time it takes for this to occur provides essential information about the efficiency and speed of the entire clotting process.
When the Prothrombin (PT) test is administered to evaluate patients taking blood-thinning medications, prothrombin time is presented as the international normalized ratio (INR). Although the PT result may vary from lab to lab for technical reasons, the INR is standardized and reproducible and is therefore the only measurement that should be used to guide warfarin dosage adjustments.
The INR Testing Procedure
Before undergoing the INR test, patients should always inform their doctors of any prescribed or over-the-counter medications, vitamins, supplements or herbal remedies they are taking, because these products may interfere with the accuracy of test results and may affect the INR, itself.
During the INR test, the healthcare professional takes a sample of the patient's blood, which is collected in a sterile vial. Chemicals are then added to the blood sample, and the clotting time of the plasma is precisely measured. Apart from the sting of needle insertion, the INR testing procedure is not uncomfortable. As long as it is performed under sterile conditions, there is no risk involved.
INR Test Results
Patients taking blood-thinning medications are presented with the international normalized ratio (INR). For those on blood thinners, the effective range is usually 2.0 to 3.0. Patients whose reading is higher have blood that clots too slowly; patients who reading is lower have blood that clots too quickly. An INR range of 2.0 to 3.0 is generally effective for people taking warfarin who need full anti-coagulation, but it may need to be slightly higher in other situations such as for patients with mechanical heart valves.
Although the INR presentation is useful for people on warfarin, it is NOT applicable to patients on other types of oral blood thinners such as Eliquis, Xarelto or Pradaxa. It also cannot be used to measure the degree of blood thinning for injectable blood thinners such as heparin or Lovenox (enoxaparin).