High INR: Causes and What to Do

Lab Tests Online UK explains that the INR is a standardised version of the prothrombin time (PT). The PT measures, in seconds, how long a blood sample takes to clot. The INR converts that into a ratio that can be compared between laboratories and countries, so that the effect of warfarin can be measured consistently.

It is used mainly to monitor warfarin and similar vitamin K antagonist tablets. The NHS says that while on warfarin you will need regular INR blood tests, that you will have them every day or every few days at the start, and usually only every 12 weeks once the dose is stable. Some people can check their INR themselves at home.

The INR can also be raised in people who are not on any anticoagulant. Lab Tests Online UK says PT or INR may be used to check for a bleeding disorder, liver disease or vitamin K deficiency, or to check clotting before surgery.

For anyone on warfarin, the aim is an INR inside an individual range, commonly 2.0 to 3.0. Lab Tests Online UK describes this as an ideal compromise between reducing clot formation and being safe with respect to bleeding. A result above the range means the dose is giving more anticoagulation than intended, and the clinic that manages your warfarin will normally adjust the dose or advise on timing.

Lab Tests Online UK lists the usual reasons for results to vary: other drugs, the onset of illness, changes in diet or weight, large amounts of alcohol, cranberry juice or grapefruit juice, and a missed or forgotten dose. It says that antibiotics and painkillers can increase PT and INR, whereas vitamin K, whether in a multivitamin or a liquid supplement, can decrease them.

The NHS adds that the medicines that may not mix well with warfarin include anti-inflammatory painkillers such as ibuprofen and aspirin, antibiotics, antidepressants, some heart medicines, medicines for stomach ulcers and fungal infections, and statins. It also advises against large amounts of alcohol, cranberry products and grapefruit juice.

An unexpectedly prolonged PT or INR in someone not taking an anticoagulant needs a reason. Lab Tests Online UK says additional testing may be necessary to determine the cause. The same source links a prolonged PT with a normal aPTT to liver disease, decreased vitamin K and decreased or defective factor VII, and a prolonged PT together with a prolonged aPTT to factor deficiencies, liver disease or disseminated intravascular coagulation.

The reason the liver features is that it makes most of the clotting proteins. Lab Tests Online UK describes the PT as one of the more sensitive tests for serious liver disease, which is the stage at which the liver loses the ability to make essential proteins including clotting factors.

Vitamin K deficiency produces a similar result because several clotting factors depend on the vitamin. A clinician will look at liver tests, nutrition, medicines and the rest of the clotting screen before choosing the next step.

NICE Clinical Knowledge Summaries explains that there are five oral anticoagulants licensed in the UK: warfarin, apixaban, dabigatran, edoxaban and rivaroxaban. Warfarin is the one that works by inhibiting vitamin K-dependent clotting factors, which is why its effect is tracked with the INR. The other four, the direct oral anticoagulants, act on factor Xa or thrombin directly.

The summary states that unlike warfarin, direct oral anticoagulants do not require regular INR monitoring, although regular follow-up is still needed to review treatment, check for bleeding and clotting events and give advice. If you take one of these medicines and have been told your INR is high, that is a reason to ask the prescriber why it was measured and what it is thought to reflect.

The same source notes that bleeding is the most common adverse effect of anticoagulants, and that warfarin, apixaban, dabigatran and rivaroxaban have antidotes that can reverse their effect, whereas there is currently none for edoxaban.

Do not adjust your own warfarin dose in response to a number. Lab Tests Online UK says the dose is adjusted by the healthcare team to bring the INR into the range right for you, and that it is not ideal to change the dose rapidly or very frequently, except when finding a starting dose.

If you have taken more than your prescribed dose, the NHS advises getting help from NHS 111. It also says to call 111 if you are on warfarin and have symptoms of bleeding, have had a head injury or think you may be having another serious side effect. For severe bleeding or signs of bleeding in the brain, such as a sudden bad headache with confusion, slurred speech or weakness, this is an emergency.

The sources read here do not give an INR at which a particular action is taken, because that is decided individually. Your anticoagulant clinic will have told you whom to contact and when.

• Tell the clinic about any new medicine, antibiotic course, illness, change in diet or heavy drinking.

• Carry your anticoagulant alert card and show it before any medical or dental procedure.

• Keep a note of missed doses and tell the clinic if you often miss them.

• Seek help urgently for blood in urine or stool, vomiting or coughing blood, a head injury or a sudden severe headache.