INR (international normalised ratio): targets, testing and warfarin

The INR is derived from the prothrombin time. Because laboratories use slightly different reagents, the raw PT in seconds varies from one laboratory to another. The INR corrects for this by comparing the patient's PT with that of a normal sample under standardised conditions. Lab Tests Online UK calls it a highly controlled version of the PT whose results are exactly reproducible no matter which laboratory or country performs the test.

The higher the INR, the longer the blood takes to clot. That is why it is used to measure the effect of warfarin, which works by inactivating vitamin K and so reducing the activity of several vitamin K-dependent clotting factors.

People on anticoagulant drugs usually have a target INR of 2.0 to 3.0, which Lab Tests Online UK describes as roughly two to three times as long as in someone not on warfarin with normal clotting. For some people at high risk of clot formation the target is higher, about 3.0 to 4.0. The range can be lower or higher depending on individual circumstances, and your anticoagulant clinic will tell you yours.

The target is a compromise. A higher INR makes a clot less likely but a bleed more likely, so the range is chosen for the underlying reason for treatment, such as atrial fibrillation, a mechanical heart valve or a clot in the leg or lung. The NHS lists these among the reasons warfarin is used.

The NHS says that while taking warfarin you need regular INR blood tests to check you are on the right dose, with tests every day or every few days when you first start and usually only every 12 weeks once the dose is stable, although some people need them more often. Sometimes you may be able to check your INR yourself at home.

Lab Tests Online UK explains that point-of-care testing uses a drop of blood from the fingertip, can give an immediate result and is more commonly done in the clinic or pharmacy. Accuracy can be harder to guarantee at home, so ask your clinic if self-monitoring is appropriate. Ideally warfarin is taken in the evening so the dose can be adjusted the same day as the test.

Lab Tests Online UK points out that many factors affect results: medicines, the onset of illness, changes in diet, weight loss or gain, large amounts of alcohol, cranberry or grapefruit juice, and missed or forgotten doses. Foods rich in vitamin K, such as liver, green tea, cabbage, kale, broccoli, turnip greens, chickpeas and soya products, can alter results if eaten in excess.

The NHS adds that antibiotics, anti-inflammatory painkillers, antidepressants, some heart, stomach ulcer and antifungal medicines and statins may not mix well with warfarin, that St John's wort and vitamin K supplements should be avoided, and that you should talk to your doctor before making big changes to your diet.

An occasional result just outside the range is not always a reason to change the dose. Warfarin affects several clotting factors that fall at different rates, and the INR reflects a standard state only after about five days on the same dose. Clinicians may stay with a dose that usually works if control is generally very good.

The NHS says the main side effect is an increased risk of bleeding. Serious signs include blood in the urine or stool, unexplained bruising or nosebleeds, coughing up or vomiting blood, and a sudden bad headache with confusion, slurred speech or difficulty moving the limbs. Call NHS 111 if you have bleeding symptoms or a head injury while on warfarin, and call 999 for severe symptoms such as these signs of bleeding in the brain.

Carry your anticoagulant alert card and show it to doctors and dentists before procedures. If you miss a dose, take it as soon as you remember; if you do not remember until the next day, skip it and never double up. If you take more than prescribed, get advice from NHS 111.

The INR is also used in people not on anticoagulants to help assess vitamin K deficiency and serious liver disease. A raised INR there shows that clotting factor production is impaired and prompts further investigation rather than a dose change.