Also Known As
Partial Thromboplastin Time
Kaolin Cephalin Clotting Time (KCCT)
Formal Name
Activated Partial Thromboplastin Time (aPTT)
This article was last reviewed on
This article waslast modified on 22 February 2023.
At a Glance
Why Get Tested?

As part of an investigation of a bleeding or thrombotic episode. To help evaluate your risk of excessive bleeding prior to a surgical procedure although numerous studies have shown that it cannot accurately estimate risk of bleeding in all patients. To monitor unfractionated heparin anticoagulant therapy.

When To Get Tested?

When you have unexplained bleeding or thrombosis (a blood clot). When you are on unfractionated or intravenous (IV) heparin anticoagulant therapy. Sometimes as part of a pre-surgical screen.

Sample Required?

A blood sample is taken by needle from a vein in the arm

Test Preparation Needed?

No test preparation is needed; however, eating a high fat meal prior to the blood test may cause interference with the test and should be avoided. It is important that the blood is collected from a medium to large calibre vein without the application of a tourniquet for a prolonged period of time to get an accurate result. The blood sample should be collected into a sodium citrate blood tube to avoid activation of clot formation prior to testing. The correct volume of blood is required for an accurate result.

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

The activated partial thromboplastin time (aPTT or PTT) measures the length of time (in seconds) that it takes for clotting to occur when specific reagents are added to plasma (liquid portion of the blood) in a test tube. It is a measure of the functionality of the intrinsic and common pathways of the coagulation cascade. The body uses the coagulation cascade to produce blood clots to seal off injuries to blood vessels and tissues, to prevent further blood loss, and to give the damaged areas time to heal. The cascade consists of a group of coagulation factors. These proteins are activated sequentially along either the extrinsic (tissue related) or intrinsic (blood vessel related) pathways. The branches of the pathway then come together into the common pathway, and complete their task with the formation of a stable blood clot. When a person starts bleeding, these three pathways have to work together.

Each component of the coagulation cascade must be functioning properly and be present in sufficient quantity for normal blood clot formation. If there is an inherited or acquired deficiency in one or more of the factors, or if the factors are functioning abnormally, then stable clot formation will be inhibited and excessive bleeding and/or clotting may occur. If factors in the common or intrinsic pathways are affected, it will cause the aPTT to be prolonged.

Accordion Title
Common Questions
  • How is it used?

    The aPTT test is used when someone has unexplained bleeding or clotting. Along with the PT test (which evaluates the extrinsic and common pathways of the coagulation cascade), the aPTT is often used as a starting place when investigating the cause of a bleeding or thrombotic (blood clot) episode. It is often used with recurrent miscarriages which may be associated with anticardiolipin or antiphospholipid antibodies. The aPTT and PT tests are also sometimes used as pre-surgical screens for bleeding tendencies, although numerous studies have shown that they are not useful for this purpose.

    The aPTT is also used to monitor heparin anticoagulant therapy. However it cannot monitor therapy with newer "low molecular weight heparin" (LMWH). LMWH effect is usually predictable from the dose and only occasionally needs to be measured, using an anti-Xa test.

    If the aPTT is prolonged, and the cause is not due to heparin contamination or to other pre-analytical problems such as an insufficient or clotted blood sample, then the aPTT is followed by mixing studies to decide whether there is deficient coagulation factor(s) or a coagulation inhibitor is present. Mixing is a procedure where the patient's plasma is mixed with pooled normal plasma (a combination of blood from different donors that has normal amounts of all of the clotting factors in it). If the patient has a factor deficiency, mixing it with the pooled normal plasma should provide enough of the missing factor(s) for the aPTT to "correct" - to clot within the normal time frame. If it does correct, specific coagulation factor testing is done to determine which factor(s) is deficient. If it does not correct, then the prolonged aPTT may be due to a specific or non-specific inhibitor. Further testing may then be done to check for antibodies to specific factors or to identify non-specific inhibitors occurring in antiphospholipid syndrome.

  • When is it requested?

    The aPTT may be requested, along with other tests such as a PT test, when a patient presents with unexplained bleeding or bruising, or a thromboembolism (blood clot). It may be useful to look for complications of some diseases, such as disseminated intravascular coagulation (DIC). DIC can occur in severe infections or some cases of cancer, causing both bleeding and clotting as coagulation factors are activated and used up at a rapid rate. The APTT may also increase in liver disease, as the liver is the source of most coagulation factors. When the patient has had a thrombotic episode or recurrent miscarriages, the aPTT may be requested as part of an evaluation for antiphospholipid syndrome.

    It may be used as part of a pre-surgical evaluation for bleeding tendencies, especially if the surgery carries an increased risk of blood loss and/or if the patient has a clinical history of bleeding - such as nosebleeds and bruising that may indicate the presence of an inherited or acquired factor deficiency or of an acquired inhibitor.

    When a patient is on intravenous (IV) or injection heparin therapy, the aPTT is ordered at regular intervals to monitor the degree of anticoagulation. When a person is switched from heparin therapy to longer-term warfarin therapy, the two are overlapped and both the aPTT and PT are monitored until the patient has stabilised.

  • What does the test result mean?

    A prolonged aPTT usually means that clotting is taking longer to occur than expected (but is associated with increased risk of blood clots if due to a lupus anticoagulant) and may be caused by a variety of factors (see the list below). Often, this suggests that there may be a coagulation factor deficiency or a specific or non-specific inhibitor affecting the body’s clotting ability. Coagulation factor deficiencies may be acquired or inherited. The majority of coagulation factors are produced in the liver and several are vitamin K dependent. Therefore if a person has liver disease or vitamin K deficiency they may have one or more factor deficiencies. Inherited factor deficiencies usually affect a single factor and may involve the quantity and/or function of the factor produced.

    Inhibitors may be antibodies that specifically target certain coagulation factors, such as Factor VIII antibodies, or they may be non-specific inhibitors, such as lupus anticoagulants that bind to chemicals called phospholipids found on the surface of platelets. Since phospholipids assist in the clotting process, and since the aPTT test reagents (chemicals used to run the tests) contain phospholipids, such antibodies may prolong the aPTT even though they are usually associated with thrombosis instead of bleeding.

    The administration of Heparin will also prolong an aPTT, either as part of anticoagulation therapy or as a contaminant. Heparin is a drug that is given intravenously (IV) or by injection to prevent and to treat thromboembolism (blood clots that block blood vessels). When it is given in therapeutic doses it must be monitored – too much and the patient will bleed excessively, too little and the patient may continue to clot. The aPTT is used to monitor heparin therapy and occasionally to monitor other therapeutic anticoagulants such as hirudin. Prolonged aPTT tests may be due to :

    1. Pre-analytical problems. These may include:
      • Insufficient sample - there must be enough blood collected. The anticoagulant to blood ratio must be 9:1
      • Patients with high or low haematocrit levels may have an altered aPTT
      • Heparin contamination. This is the most common problem - especially when blood is collected from intravenous lines that are being kept "open" with heparin flushes
    2. Inherited or acquired factor deficiencies. Factor deficiencies (Factor XI, IX, VIII, X, II, I) cause bleeding, others - contact factors (e.g. factor XII) - prolong the aPTT in vitro but do not cause bleeding and have little clinical significance. Prolonged aPTTs due to factor deficiencies usually "correct" after being mixed with pooled normal plasma.
    3. A non-specific inhibitor such as the lupus anticoagulant (LA). If the LA does prolong the aPTT, LA sensitive aPTT or Dilute Russell Viper Venom Test (DRVVT) it will not correct with normal plasma mixing but it will usually correct if an excess of phospholipid is added to the sample.
    4. A specific inhibitor. Although these are relatively rare, these are antibodies that attack a particular factor. They may develop in someone with a bleeding disorder who is receiving factor replacements (such as Factor VIII which is used to treat haemophilia A) or spontaneously as an autoantibody. The specific inhibitor will prolong the aPTT and it will not correct with mixing.
    5. Unfractionated Heparin anticoagulant therapy (the target aPTT is usually 1.5 to 2.5 times that of a "normal" control sample (referred to as the "aPTT Ratio").
    6. Warfarin anticoagulation therapy. The aPTT is relatively insensitive to warfarin and not used to monitor therapy, but it may be affected by it.
    7. Increased aPTT levels may also be seen with von Willebrand’s disease, which can lead to reduced levels of Factor VIII.
    8. Supratherapeutic doses of LMWH can prolong the aPTT.

    A decreased aPTT may result when coagulation Factor VIII is elevated. This may occur during an acute phase reaction - the blood's reaction to acute tissue inflammation or trauma. This is usually a temporary change that is not monitored with the aPTT. When the condition causing the acute phase reaction is resolved, the aPTT will return to normal levels.

    Normal aPTTs may reflect normal clotting function but moderate single factor deficiencies may still exist. They will not be reflected in the aPTT until they have decreased to 30 to 40% of normal. Also some lupus anticoagulants may be present that will not be prolonged with the usual aPTT result. If a lupus anticoagulant (LA) is suspected, an LA-sensitive aPTT can be used to test for it, or a similar test called the dilute Russell Viper Venom test (DRVVT).

  • Is there anything else I should know?

    Once heparin is started, the laboratory work-up of an abnormal aPTT is difficult. Often when a patient presents with unexplained bleeding or clotting, an aPTT will be ordered along with other bleeding and hypercoaguability tests before treatment is begun. If this is not feasible, some investigational testing may have to wait until the current condition has resolved.

    Other testing that may be done along with an aPTT includes: Prothrombin Time (PT); measures deficiencies of the extrinsic and common coagulation pathways. Comparison of aPTT and PT can give your healthcare team information as to the cause of a bleeding problem as shown in the table below.

    PT result aPTT result Possible conditions present
    Prolonged Normal Liver disease, decreased vitamin K, decreased or defective factor VII
    Normal Prolonged Decreased or defective factor VIII, IX, XI or XII, von Willebrand disease, or lupus anticoagulant present
    Prolonged Prolonged Decreased or defective factor I, II, V or X, liver disease, disseminated intravascular coagulation (DIC)
    Normal Normal Decreased platelet function, thrombocytopenia, factor XIII deficiency, mild deficiencies in other factors, mild form of von Willebrand’s disease, weak collagen

    Other testing that may be done along with an aPTT includes: Platelet counts – which should always be monitored during heparin therapy to detect heparin-induced thrombocytopenia (HIT) promptly; Dilute Russell Viper Venom Test – to investigate for lupus anticoagulants; Reptilase test – an aPTT-like test unaffected by heparin used to confirm this as the cause of an abnormal aPTT; Thrombin time testing – this is sometimes ordered to help rule out heparin contamination; Fibrinogen testing, which may be done to rule out hypofibrinogenaemia as a cause of aPTT prolongation.

  • Is the aPTT always used to monitor heparin therapy?

    In a couple of situations it is not.
    1. When very high doses of heparin are used, as may occur during open heart surgery, the aPTT loses its sensitivity - it will not clot. At this intense level of anticoagulation the Activated Clotting Time (ACT) is used as a monitoring tool instead of aPTT.
    2. LMWH (low molecular weight heparin), which is a newer form of heparin cannot be monitored by the aPTT, but does not usually require monitoring. If it does, an anti-Xa assay can be used.

  • Should everyone have their aPTT checked?

    This is not usually necessary. The aPTT is not used as a routine general screening test. It is used when someone has symptoms of abnormal bleeding or clotting. Asymptomatic patients are usually only screened prior to surgery - and then only if their doctor believes it is necessary to evaluate their risk of excessive bleeding during the procedure.

  • How can I change my aPTT?

    The aPTT is not something you can change through lifestyle changes (unless perhaps you have a vitamin K induced factor deficiency). It is a reflection of the integrity of your clotting system. If your aPTT 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 within a short time period.