Dr Leigh Martens

General Practitioner

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Blood-tests Explained


Disclaimer:

This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

INR


At a Glance

Why Get Tested?

A prothrombin time (PT) is a test used to help detect and diagnose a bleeding disorder or excessive clotting disorder; the international normalized ratio (INR) is calculated from a PT result and is used to monitor how well the blood-thinning medication (anticoagulant) warfarin (Coumadin®) is working to prevent blood clots.

When to Get Tested?

When you are taking warfarin or when you have unexplained or prolonged bleeding or inappropriate blood clotting

Sample Required?

A blood sample drawn from a vein in your arm; sometimes blood from a fingerstick

Test Preparation Needed?

None needed, although if you are receiving anticoagulant therapy, the specimen should be collected before taking your daily dose.

The Test Sample

What is being tested?

The prothrombin time (PT) is a test that helps evaluate a person's ability to appropriately form blood clots. The international normalized ratio or INR is a calculation based on results of a PT that is used to monitor individuals who are being treated with the blood-thinning medication (anticoagulant) warfarin (Coumadin®).

A PT measures the number of seconds it takes for a clot to form in a person's sample of blood after substances (reagents) are added. The PT is often performed with a partial thromboplastin time (PTT) and together they assess the amount and function of proteins called coagulation factors that are an important part of proper blood clot formation.

In the body, when there is an injury and bleeding occurs, the clotting process called hemostasis begins. This process involves in part a series of sequential chemical reactions called the coagulation cascade, in which coagulation or "clotting" factors are activated one after another and result in the formation of a clot. There must be a sufficient quantity of each coagulation factor, and each must function properly, in order for normal clotting to occur. Too little can lead to excessive bleeding; too much may lead to excessive clotting.

In a test tube during a laboratory test, there are two "pathways" that can initiate clotting, the so-called extrinsic and intrinsic pathways. Both of these then merge into a common pathway to complete the clotting process. The PT test evaluates how well all of the coagulation factors in the extrinsic and common pathways of the coagulation cascade work together. Included are: factors I (Fibrinogen), II (Prothrombin), V, VII and X. The PTT test evaluates those protein factors that are part of the intrinsic and common pathways: XII, XI, IX, VIII, X, V, II (prothrombin), and I (fibrinogen) as well as prekallikrein (PK) and high molecular weight kininogen (HK). The PT and PTT evaluate the overall ability to produce a clot in a reasonable amount of time and, if any of these factors are deficient in quantity or not functioning properly, the test results will be prolonged.

The PT test is usually measured in seconds and is compared to a normal range that reflects PT values in healthy individuals. Because the reagents used to perform the PT test vary from one laboratory to another and even within the same laboratory over time, the normal ranges also will fluctuate. To standardize results across different laboratories in the U.S. and the world, a World Health Organization (WHO) committee developed and recommended the use of the Internationalized Normalized Ratio (INR), calculated based on the PT test result, for people who are receiving the anticoagulant warfarin (Coumadin®).

The INR is a calculation that adjusts for changes in the PT reagents and allows for results from different laboratories to be compared. Most laboratories report both PT and INR values whenever a PT test is performed. The INR should be only applicable, however, for those taking the blood-thinning medication warfarin.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm or, sometimes, from a fingerstick.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed. If a person is receiving anticoagulant therapy, the specimen should be collected before the daily dose is taken.

The Test

How is it used?

The prothrombin time (PT) is used, often along with a partial thromboplastin time (PTT), to help diagnose the cause of unexplained bleeding or inappropriate blood clots. The international normalized ratio (INR) is a calculation based on results of a PT and is used to monitor individuals who are being treated with the blood-thinning medication (anticoagulant) warfarin (Coumadin®).

Several proteins called coagulation factors are involved in the process that the body uses to form blood clots to help stop bleeding (hemostasis). When an injury occurs and bleeding begins, some coagulation factors are activated in a sequence of steps (coagulation cascade) that eventually help to form a clot. There must be a sufficient quantity of each coagulation factor, and each must function properly, in order for normal clotting to occur. Too little can lead to excessive bleeding; too much may lead to excessive clotting. (See the "What is being tested?" section for more on this.)

The PT and INR are used to monitor the effectiveness of the anticoagulant warfarin. This drug affects the function of the coagulation cascade and helps inhibit the formation of blood clots. It is prescribed on a long-term basis to people who have experienced recurrent inappropriate blood clotting. The goal of warfarin therapy is to maintain a balance between preventing clots and causing excessive bleeding. This balance requires careful monitoring. The INR can be used to adjust a person's drug dosage to get the PT into the desired range that is right for the person and his or her condition.

Warfarin may be prescribed for conditions such as:

  • Irregular heartbeat (atrial fibrillation)
  • The presence of artificial heart valves
  • Deep vein thrombosis (DVT), pulmonary embolism (PE)
  • Antiphospholipid syndrome
  • Occasionally, in heart attacks with certain risk factors

The PT test may be used along with a PTT as the starting points for investigating excessive bleeding or clotting disorders. The PT evaluates the coagulation factors VII, X, V, II and I (fibrinogen). The PTT test evaluates coagulation factors XII, XI, IX, VIII, X, V, II (prothrombin), and I (fibrinogen) as well as prekallikrein (PK) and high molecular weight kininogen (HK). By evaluating the results of the PT and PTT together, a health practitioner can gain clues as to what bleeding or clotting disorder may be present. These tests are not diagnostic by themselves but usually provide information on whether further tests may be needed.

Examples of other testing that may be done along with a PT and PTT or in follow up to abnormal results include:

  • Platelet count – to determine if platelets are decreased, which can cause excessive bleeding
  • Thrombin time testing – sometimes ordered to help rule out a fibrinogen abnormality
  • Fibrinogen testing – may be done to rule out a low level or dysfunction of fibrinogen as a cause of a prolonged PT
  • Coagulation factor tests – these measure the activity (function) of coagulation factors. They can detect reduced levels of the protein or proteins that don't work properly (have reduced function). Rarely, the antigen level (quantity) of a coagulation factor may also be measured.
  • von Willebrand factor – sometimes ordered to help determine if von Willebrand disease is the cause of a prolonged PTT

Based on carefully obtained patient histories, the PTT and PT tests are sometimes selectively performed as pre-surgical or before other invasive procedures to screen for potential bleeding tendencies.

When is it ordered?

A PT and INR are ordered on a regular basis when a person is taking the anticoagulant drug warfarin to ensure that the prescription is working properly and that the PT/INR is appropriately prolonged. There is no set frequency for doing the test. A health practitioner will order them often enough to make sure that the drug is producing the desired effect - that it is increasing the person's clotting time to a therapeutic level without significant risk of excessive bleeding or bruising.

The PT may be ordered when a person who is not taking anticoagulant drugs has signs or symptoms of excessive bleeding or clotting, such as:

  • Unexplained bleeding or easy bruising
  • Nosebleeds
  • Bleeding gums
  • A blood clot in a vein or artery
  • An acute condition such as disseminated intravascular coagulation (DIC) that may cause both bleeding and clotting as coagulation factors are used up at a rapid rate
  • A chronic condition such as severe liver disease that may affect hemostasis

PT, along with PTT, may be ordered prior to surgery when the surgery carries an increased risk of blood loss and/or when the person has a clinical history of bleeding, such as frequent or excessive nosebleeds and easy bruising, which may indicate the presence of a bleeding disorder.

What does the test result mean?

For people taking warfarin, most laboratories report PT results that have been adjusted to the INR. These people should have an INR of 2.0 to 3.0 for basic "blood-thinning" needs. For some who have a high risk of a blood clot, the INR needs to be higher - about 2.5 to 3.5.

The test result for a PT depends on the method used, with results measured in seconds and compared to the normal range established and maintained by the laboratory that performs the test. This normal range represents an average value of healthy people who live in that area and will vary somewhat from lab to lab. Someone who is not taking warfarin would compare their PT test result to the normal range provided with the test result by the laboratory performing the test.

A prolonged PT means that the blood is taking too long to form a clot. This may be caused by conditions such as liver disease, vitamin K deficiency, or a coagulation factor deficiency. The PT result is often interpreted with that of the PTT in determining what condition may be present.

Interpretation of PT and PTT in Patients with a Bleeding or Clotting Syndrome

PT resultptt resultExamples of conditions that may be present
Prolonged Normal Liver disease, decreased vitamin K, decreased or defective factor VII, chronic low-grade disseminated intravascular coagulation (DIC), anticoagulation drug (warfarin) therapy
Normal Prolonged Decreased or defective factor VIII, IX, XI, or XII, von Willebrand disease (severe type), presence of lupus anticoagulant, autoantibody against a specific factor (e.g., factor VIII)
Prolonged Prolonged Decreased or defective factor I, II, V or X, severe liver disease, acute DIC, warfarin overdose
Normal Normal or slightly prolonged May indicate normal hemostasis; however, PT and PTT can be normal in conditions such as mild deficiencies in coagulation factor(s) and mild form of von Willebrand disease. Further testing may be required to diagnose these conditions.

Is there anything else I should know?

Some consumed substances, such as alcohol, can affect the PT and INR tests. Some antibiotics can increase the PT and INR. Barbiturates, oral contraceptives and hormone-replacement therapy (HRT), and vitamin K (either in a multivitamin or liquid nutrition supplement) may decrease PT. Certain foods, such as beef and pork liver, green tea, broccoli, chickpeas, kale, turnip greens, and soybean products, contain large amounts of vitamin K and can alter PT results. It is important that a healthcare provider know about all of the drugs, supplements, and foods that a person has ingested recently so that the PT and INR results are interpreted and used correctly.

Some laboratories will report a PT as a percentage of normal, although this is not a common practice.