What is PT-INR monitoring? Cardiology Basics

What is PT-INR monitoring? Cardiology Basics

INR is short for International Normalized Ratio of Prothrombin Time. It is also called PT-INR. The INR monitoring is used to monitor the dosage of vitamin K antagonists like warfarin. Vitamin K antagonists are used for stroke prevention in atrial fibrillation as well as for deep vein thrombosis and pulmonary embolism. Non vitamin K oral anticoagulants (NOACs), also known as direct oral anticoagulants (DOACs) like dabigatran, rivaroxaban and apixaban do not need INR monitoring. Warfarin is routinely used when there is a mechanical prosthetic valve, to prevent valve thrombosis and thromboembolism. As of now, DOACs are not ideal for mechanical prosthetic valves as they have a higher thrombogenicity.

The target range of INR values needed depend on the indication for using it. In most cases the target INR value is 2-3. If the value obtained in the test while on medication is higher, the dose of the medication is reduced and vice versa. In certain cases, as with a mechanical prosthetic mitral valve, a higher INR value of 2.5 to 3.5 is recommended. In general, prosthetic valves at mitral position have a higher thrombotic risk. Newer types of aortic mechanical prosthetic valves with an INR goal of 1.5-2.5 with low dose aspirin, after the initial 3 months are also available [1].

Several other medications and foods interfere with the action of coumadin or warfarin. Highly protein bound medications displace warfarin from plasma proteins and increase bioavailability. Green leafy vegetables with high vitamin K content can reduce INR. Whenever other medications are added or removed, it is better to recheck the INR even if it was stable earlier. For routine monitoring a test once a month would be adequate. There is a concept of time in therapeutic range or TTR. Higher TTR with few values outside the target range should be our goal.

Usual INR testing is done using venous samples at a laboratory. But now devices for home INR monitoring, which can test the INR value with blood obtained from a finger prick are also available. Though these devices need an initial extra cost, they offer the convenience of INR monitoring from home, which is especially useful for those staying far away from cities where most of the labs are located. This can potentially improve compliance of INR monitoring and increase TTR.

In addition to routine monthly monitoring of INR, it is mandatory to check INR whenever any bleeding manifestation is noted so that one can reduce the dose of warfarin if the INR is high. Very high INR values, usually well above 5 are better managed in the hospital setting. It is mentioned that prothrombin complex concentrate if available, can reverse the effects of warfarin within minutes. Reversal with fresh frozen plasma can take hours while that with vitamin K takes 12 to 24 hours. Vitamin K may be needed for longer effect due to the long half life of warfarin. But caution is needed in case of mechanical prosthetic valves because it may take a longer time for getting INR in therapeutic range after administration of vitamin K, and may be better avoided in such situations.

Reference

1. Kido K, Ball J. Optimal Intensity of Warfarin Therapy in Patients With Mechanical Aortic Valves. J Pharm Pract. 2019 Feb;32(1):93-98. doi: 10.1177/0897190017734765. Epub 2017 Oct 5. PMID: 28982306.