Patient Test Information


Also known as:

Activated Partial Thromboplastin Time; aPTT; APTT

Formal name:

Partial Thromboplastin Time

Related tests:

PT and INR; Fibrinogen; D-dimer; Thrombin Time; Lupus Anticoagulant Testing; ACT; Coagulation Factors; Platelet Count; Heparin Anti-Xa; von Willebrand Factor; Antiphospholipid Antibodies

Why Get Tested?

As part of an investigation of a possible bleeding disorder or blood clot (thrombotic episode); to monitor unfractionated (standard) heparin anticoagulant therapy; as part of screening before surgery or other invasive procedure

When to Get Tested?

When you have unexplained bleeding, inappropriate blood cloting, or recurrent miscarriages; when you are on standard heparin anticoagulant therapy; sometimes as part of a pre-surgical screen

Sample Required?

A blood sample drawn by needle from a vein in your arm

Test Preparation Needed?


How is it used?

The PTT is used primarily to investigate unexplained bleeding or clotting. It may be ordered along with a prothrombin time (PT) test to evaluate hemostasis, the process that the body uses to form blood clots to help stop bleeding. These tests are usually the starting points for investigating excessive bleeding or clotting disorders.

Several proteins called coagulation factors are involved in hemostasis and the formation of blood clots. 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. (See the "What is being tested?" section for more on this.)

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

Some examples of uses of a PTT include:

  • To identify coagulation factor deficiency; if the PTT is prolonged, further studies can then be performed to identify what coagulation factors may be deficient or dysfunctional, or to determine if an antibody to a coagulation factor (a specific inhibitor) is present in the blood.
  • To detect nonspecific autoantibodies, such as lupus anticoagulant; these are associated with clotting episodes and with recurrent miscarriages. For this reason, PTT testing may be performed as part of a clotting disorder panel to help investigate recurrent miscarriages or diagnose antiphospholipid syndrome (APS). A variation of the PTT called the LA-sensitive PTT may be used for this purpose.
  • To monitor standard (unfractionated, UF) heparin anticoagulant therapy; heparin is an anticoagulation drug that is given intravenously (IV) or by injection to prevent and to treat blood clots (embolism and thromboembolism). It prolongs PTT. When heparin is administered for therapeutic purposes, it must be closely monitored. If too much is given, the treated person may bleed excessively; with too little, the treated person may continue to clot.
  • 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.

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

  • Platelet count - should always be monitored during heparin therapy to promptly detect any heparin-induced thrombocytopenia
  • Thrombin time testing - sometimes ordered to help rule out heparin contamination
  • Fibrinogen testing - may be done to rule out a low level of fibrinogen as a cause of a prolonged PTT
  • When an initial PTT is prolonged, a second PTT test is performed by mixing the person's plasma with pooled normal plasma (a collection of plasma from a number of normal donors). If the PTT time returns to normal ("corrects"), it suggests a deficiency of one or more of the coagulation factors in the person's plasma. If the time remains prolonged, then the problem may be due to the presence of an abnormal specific factor inhibitor (autoantibody) or nonspecific lupus anticoagulant.
  • 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.
  • Dilute Russell viper venom test (DRVVT) - a test that may be done if the presence of lupus anticoagulant is suspected (See the page on Lupus Anticoagulant Testing for more on this.)
  • von Willebrand factor - sometimes ordered to help determine if von Willebrand disease is the cause of a prolonged PTT

When is it ordered?

The PTT may be ordered along with other tests such as a PT when a person has:

  • Unexplained bleeding or easy bruising
  • 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 liver disease that may affect hemostasis

A PTT may be ordered:

  • When someone has had a blood clot or when a woman has had recurrent miscarriages, as part of an evaluation for lupus anticoagulant, anticardiolipin antibodies, and antiphospholipid syndrome
  • On a regular basis, when a person is on standard (unfractionated) heparin therapy; when someone is switched from heparin therapy to longer-term warfarin (Coumadin®) therapy, the two are overlapped and both the PTT and PT are monitored until the person has stabilized.
  • 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 nose bleeds and easy bruising, which may indicate the presence of a bleeding disorder

What does the test result mean?

PTT results are typically reported in seconds. A PTT result that falls within a laboratory's reference interval usually indicates normal clotting function. However, mild to moderate deficiencies of a single coagulation factor may be present. The PTT may not be prolonged until the factor levels have decreased to 30% to 40% of normal. Also lupus anticoagulant may be present but may not prolong the PTT result. If the lupus anticoagulant (LA) is suspected, a more sensitive LA-sensitive PTT or a dilute Russell viper venom time (DRVVT) can be used to test for it.

A prolonged PTT means that clotting is taking longer to occur than normal and may be due to a variety of causes. Often, this suggests that there may be a coagulation factor deficiency or a specific or nonspecific antibody (inhibitor) affecting the body's clotting ability. Coagulation factor deficiencies may be acquired or inherited.

  Prolonged PTT tests may be due to:

  • Inherited factor deficiencies:
    • von Willebrand disease is the most common inherited bleeding disorder and it affects platelet function due to decreased von Willebrand factor.
    • Hemophilia A and hemophilia B (Christmas disease) are two other inherited bleeding disorders resulting from a decrease in factors VIII and IX, respectively.
    • Deficiencies of other coagulation factors, like factors XII and XI
  • Acquired factor deficiencies:
    • An example of an acquired deficiency is one due to lack of vitamin K. Vitamin K is essential for the formation of coagulation factors. Vitamin K deficiencies are rare but can occur due to an extremely poor diet, malabsorption disorders, or prolonged use of certain antibiotics, for example.
    • Most coagulation factors are produced by the liver, thus liver disease may cause prolonged PT and PTT. With liver disease and vitamin K deficiency, PT is more likely to be prolonged than is PTT.
  • A nonspecific inhibitor such as the lupus anticoagulant–the presence of these inhibitors is usually associated with inappropriate clotting (thrombosis), but can prolong the PTT. See the individual test articles for more on this.
  • A specific inhibitor–although relatively rare, these are antibodies that specifically target certain coagulation factors, such as antibodies that target factor VIII. They may develop in someone with a bleeding disorder who is receiving factor replacements (such as factor VIII, which is used to treat hemophilia A) or spontaneously as an autoantibody. Factor-specific inhibitors can cause severe bleeding.
  • Heparin–is an anticoagulant and will prolong a PTT, either as a contaminant of the sample or as part of anticoagulation therapy. For anticoagulant therapy, the target PTT is often about 1.5 to 2.5 times longer than a person's pretreatment level.
  • Warfarin (Coumadin®) anticoagulation therapy–the PTT is not used to monitor warfarin therapy, but it may be affected by it. Typically, the PT is used to monitor warfarin therapy.
  • Other anticoagulants–anticoagulation therapy with direct thrombin inhibitor (e.g., argatroban, dabigatran) or direct factor Xa inhibitor (e.g., rivaroxaban)
  • Prolonged PTT levels may also be seen with leukemia, excessive bleeding in pregnant women prior to or after giving birth, or recurrent miscarriages.

Results of the PTT are often interpreted with that of the PT in determining what condition may be present.

PT resultptt resultCommon condition present
Prolonged Normal liver disease, decreased vitamin K, decreased or defective factor VII
Normal Prolonged Hemophilia A or B (decreased or defective factor VIII or IX) or factor XI deficiency, von Willebrand disease, factor XII deficiency, or lupus anticoagulant present
Prolonged Prolonged Decreased or defective factor I (fibrinogen), II (prothrombin), V or X, severe liver disease, disseminated intravascular coagulation (DIC)
Normal Normal or slightly prolonged
May indicate normal hemostasis; however PT and PTT can be normal in conditions such as mild deficiencies in other factors and mild form of von Willebrand disease. Further testing may be required to diagnose these conditions.

Shortened PTT tests may be due to:

  • Disseminated intravascular coagulation (DIC)–in the early stages of DIC, there are circulating procoagulants that shorten the PTT.
  • Extensive cancer (ovarian, pancreatic, colon), except when the liver is involved
  • An acute-phase reaction: this is a condition causing pronounced tissue inflammation or trauma that elevates factor VIII levels. It is usually a temporary change that is not monitored with a PTT test. When the condition causing the acute phase reaction is resolved, the PTT will return to normal.

Is there anything else I should know?

Two anticoagulants often used, low molecular weight heparin (LMWH) and danaparoid, may not prolong the PTT and, if indicated, should be monitored using the heparin anti-factor Xa assay.

Several factors can affect results of a PTT and the interpretation of test results:

  • People with high hematocrit levels may have prolonged PTTs (in vitro artifact).
  • Heparin contamination - this is the most common problem, especially when blood is collected from intravenous lines that are being kept "open" with heparin washes.
  • Drugs such as antihistamines, vitamin C (ascorbic acid), aspirin, and chlorpromazine
  • In some cases, heparin can unintentionally decrease a person's platelet count in a complication called heparin-induced thrombocytopenia. When this occurs, substitute anticoagulants such as a direct thrombin inhibitor (e.g., argatroban or bivalirudin) may be given. The PTT test is also used to monitor these therapies. It does not directly measure the anticoagulants used but measures their effect on blood clotting.

What is being tested?

The partial thromboplastin time (PTT) is a screening test that helps evaluate a person's ability to appropriately form blood clots. It 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 PTT assesses the amount and the function of certain proteins called coagulation factors that are an important part of blood clot formation.

When body tissue(s) or blood vessel walls are injured, bleeding occurs and a process called hemostasis begins. Small cell fragments called platelets adhere to and then clump (aggregate) at the injury site. At the same time, a process called the coagulation cascade begins and coagulation factors are activated. Through the cascading reactions, threads called fibrin form and crosslink into a net that adheres to the injury site and stabilizes it. Along with the platelets adhering, this forms a stable blood clot to seal off injuries to blood vessels, prevents additional blood loss, and gives the damaged areas time to heal.

Each component of this hemostatic process must function properly and be present in sufficient quantity for normal blood clot formation. If there is a deficiency in one or more of these factors, or if the factors function abnormally, then a stable clot may not form and bleeding continues.

With a PTT, a person's sample is compared to a normal reference interval for clotting time. When a person's PTT takes longer than normal to clot, the PTT is considered "prolonged." A prolonged PTT may be due to a condition that decreases or creates a dysfunction in one or more coagulation factors. Less often, it may be due to a condition in which the body produces certain antibodies directed against one or more coagulation factors, affecting their function.

Sometimes a PTT may be prolonged because the person tested produces an autoantibody called an antiphospholipid antibody that interferes with the test. This type of antibody affects the results of the test because it targets substances called phospholipids that are used in the PTT. Though antiphospholipid antibodies can prolong the PTT test result, in the body they are associated with excessive clotting. A person who produces these antibodies may be at an increased risk for a blood clot. A PTT maybe used as part of an evaluation of a person with signs and symptoms of excessive clotting or antiphospholipid syndrome. (See the article on Antiphospholipid Antibodies for additional details.)

When a PTT is used to investigate bleeding or clotting episodes, it is often ordered along with a prothrombin time (PT). A health practitioner will evaluate the results of both tests to help determine the cause of bleeding or clotting episode(s).

It is now understood that coagulation tests such as the PT and PTT are based on what happens artificially in the test setting (in vitro) and thus do not necessarily reflect what actually happens in the body (in vivo). Nevertheless, they can be used to evaluate certain components of the hemostasis system. The PTT and PT tests each evaluate coagulation factors that are part of different groups of chemical reaction pathways in the cascade, called the intrinsic, extrinsic, and common pathways. (For more on this, see the article on the Coagulation Cascade.)

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

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

No test preparation is needed; however, a high-fat meal prior to the blood draw may interfere with the test and should be avoided.

  1. Is the PTT always used to monitor heparin therapy?

    In a few situations, it is not.

    1. When very high doses of heparin are used, as may occur during open-heart surgery, the PTT loses its sensitivity; it will not clot. At this intense level of anticoagulation, the activated clotting time (ACT) can be used as a monitoring tool.
    2. Some doctors and laboratories now monitor standard (unfractionated) heparin therapy using the chromogenic anti-factor Xa test.
    3. Low molecular weight heparin (LMWH) is a fast-acting form of heparin often used in the treatment of conditions such as deep vein thrombosis (DVT) prevention. Though generally not requiring monitoring, it must be monitored using the anti-factor Xa test.
    4. For people with lupus anticoagulant and clotting and who are being treated with heparin, the PTT is not reliable; thus the anti-factor Xa assay must be used to monitor their heparin therapy.
  2. Should everyone have their PTT checked?

    The PTT is not used as a routine screening test but is ordered when someone has symptoms or a family history of abnormal bleeding or clotting. Asymptomatic people are occasionally screened prior to a surgery if their healthcare provider feels that it will help evaluate their risk of excessive bleeding during the procedure.

  3. How can I change my PTT?

    The PTT is not something you can change through lifestyle changes (unless you have a vitamin K deficiency). It is a reflection of the integrity of your clotting system. If your PTT is prolonged due to acquired factor deficiencies, then addressing the underlying condition may bring the results to near normal levels. If they are prolonged due to a temporary or acute condition, they should return to normal on their own when the acute condition is resolved. Inherited coagulation abnormalities or deficiencies must be routinely monitored and may be treated with frequent replacement infusions of the missing clotting factor.

  4. What is an LA-sensitive PTT and how does it differ from regular PTT?

    The LA-sensitive PTT (LA-PTT or PTT-LA) is a variation of the PTT used in a series of tests evaluating the presences of lupus anticoagulant, an antibody associated with clotting episodes and recurrent miscarriages. The LA-PTT uses a low phospholipid reagent that is optimized for detecting lupus anticoagulants. The test is based on the principle that lupus anticoagulant binds to the phospholipids that are used as one of the reagents in the PTT test, causing an abnormally prolonged clotting time. For more on this, see the article on Lupus Anticoagulant Testing.