Warfarin INR: Interactions, Monitoring and Reversal

Warfarin INR questions test both pharmacology and safe practice. Warfarin remains important for mechanical heart valves and some other indications, even though direct oral anticoagulants are more common for many patients. The topic is full of rules that exams like to check.

Mechanism

Warfarin inhibits vitamin K epoxide reductase, which reduces the production of vitamin K-dependent clotting factors II, VII, IX and X, as well as proteins C and S. Because proteins C and S have shorter half-lives, early treatment can cause a transient hypercoagulable state and, rarely, skin necrosis. This is why heparin is often overlapped at the start for acute thrombosis.

Monitoring

The international normalized ratio is used to adjust dosing. Most indications aim for a target of around two to three, and some, such as certain mechanical valves, aim higher. Frequent testing is needed when starting treatment, after dose changes and when new drugs are added.

Interactions

Remember the direction of change:

• Increase the INR: metronidazole, fluconazole, amiodarone, macrolides, trimethoprim-sulfamethoxazole and acute alcohol intake
• Decrease the INR: rifampicin, carbamazepine, phenytoin, St John’s wort and chronic alcohol use
• Diet: vitamin K-rich foods can reduce the effect if intake changes suddenly

Non-steroidal anti-inflammatory drugs and antiplatelet drugs increase bleeding risk even without changing the INR.

Pregnancy

Warfarin is teratogenic, and warfarin embryopathy is a classic exam fact. Heparin is used instead in pregnancy for most indications.

Managing a high INR

Management depends on the level and on bleeding. For a high INR without bleeding, hold the dose and consider low-dose oral vitamin K for very high values according to local protocol. For major bleeding, stop warfarin, give intravenous vitamin K and prothrombin complex concentrate, or fresh frozen plasma if concentrate is not available.

Other anticoagulants

Know the reversal agents for direct oral anticoagulants: idarucizumab for dabigatran, and andexanet alfa or prothrombin complex concentrate for factor Xa inhibitors, according to local availability.

A classic exam pattern

A patient stable on warfarin begins a course of metronidazole and presents with a markedly raised INR and bruising. The mechanism is reduced warfarin metabolism, and the action is to review dosing and monitor closely.

Study tips

• Create two lists: drugs that raise and drugs that lower the INR
• Memorize the reversal agents
• Review each wrong answer for the interaction you missed

This article is for exam revision and does not replace current clinical guidelines. Practice pharmacology in the Namazj free trial.