APTT

Specimen Required

Sodium Citrate (PT Vial)

Fasting Required

Purpose of the Test

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 this test is required

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 the Test Detects

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,…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. See MoreSee Less

Preparation for the Test

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.

Sample Requirements

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

Additional Notes

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.

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