INR Testing: An Introduction

INR Testing: An Introduction

  • 15 Dec 0
CouguCheck XS system

INR Testing: An Introduction


The INR is a test for blood clotting and is primarily used in monitoring warfarin therapy with the aim of maintaining an elevated INR within a certain range such as 2.0 to 3.0. This test is initially checked on a frequent basis and as treatment is stabilized, INR may be conducted less often like fortnightly. The change in warfarin dosage takes a number of days to affect INR results. INR testing is a blood test and therefore needs a small tube of blood sample from a vein-roughly 4 milliliters. It is essential that this tube be filled to the proper level, or else false results may occur.

The INR testing is usually carried out as part of warfarin treatment, but it can also be checked by a doctor with regards to Liver Function Tests, since liver dysfunction may lead to reduced production of certain clotting factors. Common reasons for warfarin treatment include deep venous thrombosis (a clot within a deep vein), pulmonary embolism (a clot in the lung), Atrial Fibrillation (an irregular heartbeat which is sometimes accompanied by enlarged left atrium) and in some cases heart failure.

The results of INR testing are given as a number. Since the number is in form of a ratio, there are no units of measurements for INR test. The ratio is of the Prothrombin Time (a measure of clotting) of the sample to the Prothrombin Time (PT) of a normal blood sample. A result of 1.0 to 1.5 is thus normal. Patients on warfarin therapy will have a different target INR range to aim for with the warfarin therapy, depending on the reasons for blood thinning treatment (anticoagulation). One example would be a range of 2.0-3.0 for DVT. An INR result lower than the desired range therefore means that the blood clots too easily or not thin enough” and an INR higher than the desired range will mean that the blood is too thin” for the patient.

The warfarin doses are adjusted, every few days, targeting the desired range of INR. While the treatment is stabilized, the dose may be given less often. Patients on warfarin therapy should consult their doctor or the laboratory performing INR testing so as to know whether they need to adjust their warfarin dose and also know what dose to take on the basis of the INR results. This result should be taken in context of the recent INR measurements and the dose changes. Most medications can affect INR results and even the change in diet could result in INR changes either lowering or raising it.

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