Prothrombin time test (PT and INR) (PL / INR)

The prothrombin time measures how long it takes for blood plasma to clot once a special reagent is added. It shows how the so-called extrinsic clotting pathway works — the pathway that depends on clotting factors made in the liver (among them II, VII, IX and X), whose production requires vitamin K. Because the result differs depending on the reagent a laboratory uses, the international normalised ratio (INR) is calculated alongside it: the INR makes results comparable between different laboratories and is especially important for people taking warfarin.

PT/INR helps to assess the risk of bleeding or of excessive clotting, to check liver function (the liver makes most of the clotting factors), to identify vitamin K deficiency and to monitor whether a warfarin dose is suitable. The test is often done together with the activated partial thromboplastin time (aPTT), which assesses the other clotting pathway.

• Before surgery or an invasive procedure — to assess the risk of bleeding
• While taking warfarin — to monitor regularly whether the dose is suitable (the target is INR 2.0–3.0)
• When there is unexplained bleeding or bruises appear more easily
• When liver disease is suspected, or when liver function is being assessed
• When vitamin K deficiency is suspected
• When blood clots form for no clear reason

A raised prothrombin time or INR (above 1.1 if no blood-thinning medicines are being taken) means the blood clots more slowly than usual, which increases the risk of bleeding. The most common causes are bleeding disorders, liver disease, vitamin K deficiency or disseminated intravascular coagulation (DIC). In people taking warfarin, too high an INR means the medicine is having too strong an effect.

• Bleeding disorders (for example a deficiency of clotting factors)
• Liver disease — the production of clotting factors is disturbed
• Vitamin K deficiency
• Disseminated intravascular coagulation (DIC)
• Too high a dose of a blood-thinning medicine (for example warfarin)

Symptoms that can accompany it: unexplained heavy bleeding, bruises that appear easily, heavy nosebleeds and unusually heavy periods.

What to discuss with a doctor: whether the result is assessed together with aPTT and liver markers (ALT, AST); whether warfarin or another blood-thinning medicine is being taken and whether the dose matches the target INR; diet and vitamin K intake, if a deficiency is suspected; and when to repeat the test to follow the change.

A shortened prothrombin time or a low INR (below 0.8) means the blood clots faster than usual, and this increases the risk of unwanted blood clots. In people taking warfarin, too low an INR most often means that the dose of the medicine is too small or that other factors have weakened its effect. Low results are less common than high ones and are usually assessed together with clinical signs.

• An unsuitable (too small) warfarin dose
• Alcohol use
• Over-the-counter medicines, vitamins or food supplements that reduce the effect of warfarin
• Antibiotics or dietary changes that alter the effect of the medicine

Symptoms that can accompany it: pain or swelling in the leg, redness or warmth of the skin on the leg, difficulty breathing, coughing, chest pain, a fast heartbeat or fast breathing.

What to discuss with a doctor: whether the warfarin dose is being taken correctly and whether diet or other medicines have changed; clinical signs of thrombosis — leg swelling, pain on breathing; when to repeat the test so that the effect of a dose adjustment can be seen; and aPTT and other clotting markers if symptoms persist.

Related indicators

• aPTT — the other clotting pathway
• D-dimer
• Fibrinogen
• Platelets
• ALT — a liver marker
• Vitamin K