INR

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.

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.

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

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.