INR blood test and warfarin explained

What does the INR measure?

INR stands for international normalised ratio. It is a standard blood test that measures how long the blood takes to clot. One NHS trust explains that it compares the clotting time of a person taking warfarin with that of someone who is not.

Why is the INR checked on warfarin?

Warfarin does not have a fixed dose. The dose needed depends on how much the medicine slows the clotting of the blood, and the INR test measures that effect. The main side effect of warfarin is an increased risk of bleeding. One NHS trust leaflet explains that a typical target INR means the blood takes just over twice as long as normal to clot.

How often is it done?

At the start, testing is frequent. Once the result is stable, the gaps between tests lengthen. Some people need tests more regularly than others. The INR results go to the warfarin clinic, which advises on the ongoing dose and the date of the next test. One NHS trust leaflet says that once the dose has stabilised, blood tests become less frequent, and your anticoagulant clinic decides how often you need one.

What can change the INR?

New medicines can change the effect of warfarin, so one NHS service asks for an INR test soon after any new medicine is started or stopped. Alcohol, diet and other medicines can also move the result. Sources name cranberry and grapefruit juice as drinks to avoid on warfarin, and binge drinking as able to raise the INR. They also name ibuprofen and similar painkillers as able to increase the risk of bleeding. The NHS advises telling your doctor about any other medicines, herbal remedies, vitamins or supplements, checking before starting any new medicine, and avoiding St John's wort. Guidance warns that binge drinking while taking warfarin is dangerous because it can raise the INR and the risk of bleeding.

What should people know about bleeding?

Signs of bleeding include bleeding gums, blood in vomit, blood in urine or stool, and prolonged bleeding from small cuts. For women, heavier periods can be a sign. NHS trust leaflets say to contact the GP or NHS 111 about minor bruising or bleeding. NHS leaflets say to call 999 if bleeding cannot be stopped. NHS trust leaflets list vomiting or coughing up blood as a reason to seek care at A&E. NHS guidance says a significant blow to the head is a reason to call 999.

What else do people on warfarin carry or watch?

People on warfarin are given an anticoagulant alert card to carry. The card lets medical staff identify that someone takes warfarin if there is an accident. Foods high in vitamin K, such as leafy green vegetables, can affect how warfarin works. Dietary advice from an NHS trust stresses eating a steady amount of such foods, because the clinic adjusts the dose to match. The NHS says the food and drink restrictions do not usually apply to the newer anticoagulants such as apixaban, rivaroxaban, edoxaban and dabigatran. The NHS says to carry the anticoagulant alert card at all times and show it to your doctor or dentist before any medical or dental procedure. NHS dietary leaflets say that a big increase or decrease in foods containing vitamin K from week to week can affect the INR, so it is best to keep intake steady and talk to your anticoagulant clinic before major diet changes.