INR Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Bogna Szyk and Jack Bowater
Based on research by
Riley RS, Rowe D, Fisher LM. Clinical utilization of the international normalized ratio (INR) Journal of Clinical Laboratory Analysis (April 2000)See 1 more source
Khatib R, Ludwikowska M, Witt DM, Ansell J, Clark NP, Holbrook A, Wiercioch W, Schünemann H, Nieuwlaat R. Vitamin K for reversal of excessive vitamin K antagonist anticoagulation: a systematic review and meta-analysis Blood Advances (March 2019)
Last updated: Sep 23, 2022

The International Normalized Ratio (INR) calculator is a simple, free tool to assess the unified prothrombin time (PT), and its meaning. It's an easy way to evaluate both the extrinsic pathway and the common pathway of coagulation.

If you'd like to find out what is the INR normal range and how to calculate it, or discover why your INR results are so important - read on! 🧮

What does INR measure?

In general, INR informs us about blood's ability to clot. It is used to measure liver function, effects of certain drugs (older anticoagulants, see: warfarin dosing calculator), and the levels of vitamin K.

INR (International Normalized Ratio) is an analytic calculation created to unify the prothrombin time (PT) acquired in different laboratories using different reagents. To put it simply: because of the creation of INR, all Polish, American, and Chinese doctors can talk about the PTs of their patients together without getting confused with its various values.

To be precise, INR describes both the extrinsic pathway and common pathway of coagulation, that is, it allows us to measure the levels of different clotting factors, enumerated below:

  1. Extrinsic pathway:
  • VII (proconvertin).
  1. Common pathway:
  • I (fibrinogen);
  • II (prothrombin);
  • V (proaccelerin); and
  • X (Stuart–Prower factor).

Other methods used to measure the clotting of the blood are:

INR result meaning

The higher the INR, the longer it takes for the blood to clot. Elevated INR may inform us of an increased risk of bleeding, while low INR can mean a possibility of thromboembolism complications.

  1. A PT INR test is a simple and efficient way of controlling the treatment of people that suffer from atrial fibrillation, mechanical heart valves, thromboembolism, stroke, or any disease of the peripheral arteries:
  • INR < 1.1 - the average level for a healthy person, who not currently being treated with any anticoagulation drugs;
  • 1.1 < INR < 2.0 - an elevated level of INR, not yet sufficient to diagnose patient being treated with anticoagulation drugs. A possible sign of a liver insufficiency;
  • 2.0 ≤ INR ≤ 3.0 - the therapeutic level for people treated with warfarin & other, older types of anticoagulants; and
  • INR > 3.0 - the INR is too high and may pose the risk of excessive bleeding❗This level of INR might be recommended for people who have the older generation of artificial heart valve.
  1. Other reasons for higher INR:
  • Inherited or acquired disease that causes the deficiency of one of the clotting factors (I, II, V, VII, X);
  • Disseminated intravascular coagulation (DIC);
  • Depletion/insufficient intake of vitamin K; and
  • Destruction of the liver, caused by, e.g., cirrhosis.

💡 Anticoagulants are drugs designed to slow down the clotting of the blood. Their effect is shown by an increased PT or INR. New oral anticoagulants, such as dabigatran, do not require regular check-ups and blood tests, making them more comfortable to use.

We always need to think about the possible complications & benefits connected to any treatment. To assess the risk of diseases that should be treated with anticoagulants, we can use one of the tools enumerated below:

Risk of thromboembolism:

Risk of pulmonary embolism:

How to use the INR calculator?

Our INR calculator needs three types of data:

  1. The patient's prothrombin time (PT) in seconds;

PT is one of the standard blood tests performed for the assessment of blood clotting.
2. The PT of the control, standard sample used in your local diagnostic center, in seconds; and
3. The ISI (International Sensitivity Index) value, determined by a given manufacturer. It measures the sensitivity of the tissue factor used in the PT analytic process.

Or INR calculator will perform a quick INR check, and display the significance of your results.

How to calculate INR from PT?

The equation used in our INR calculator looks as follows:

INR =(Patients PT/Control PT)ISIINR\ = (Patient's\ PT / Control\ PT)^{ISI}

ISI usually takes the values of 0.9-1.7. Both control PT and ISI depend on the analytical method used, and should be well known and distributed by your local laboratory.

The INR test should be performed in case of any suspicion of a coagulation defect. It is also crucial to know your patient's INR/PT status before any non-elective surgery.


What are the symptoms of high INR levels?

A high INR level is usually asymptomatic, but if the symptoms appear, they can be:

  • Nosebleeds, bleeding gums, heavy periods in women;
  • Vomiting that looks like coffee grounds, black stools;
  • Headache, confusion (due to intracranial bleeding);
  • Spontaneous bruising;
  • Abdominal pain (in case of internal bleeding to the abdomen);
  • Chest pain; and
  • Joints and muscles pains.

How do I calculate protime?

Protime, also known as prothrombin time or PT, is not really calculated. It is an actual time, measured in seconds in the laboratory. The scientist takes a patient's blood sample and adds a set of reagents to make the blood clot. Then they measure how long it takes.

What is ISI in PT INR?

ISI stands for International Sensitivity Index. When PT (prothrombin time) or INR (international normalized ratio) is measured in the laboratory, a set of blood-clotting reagents is needed. ISI is a parameter that describes this set of reagents in terms of sensitivity. ISI is usually given by the manufacturer of the reagents.

How do I calculate INR while PT is 22 seconds?

To calculate INR:

  1. Let the patient's PT equal 22 seconds, the control PT equal 12, and ISI equal 1.3.
  2. Divide the patient's PT by the control PT. This yields 1.83.
  3. Raise this number by a factor of the ISI, giving 2.2.
  4. In this case, INR equals 2.2.
Łucja Zaborowska, MD, PhD candidate
Patient's PT
Control PT
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