Partial thromboplastin time (aPTT or PTT)
What is analysed?
The activated partial thromboplastin time (aPTT or PTT) is a laboratory test that measures the time (in seconds) that it takes for blood clotting to occur when specific reagents are added to plasma (the liquid component of blood) in a test tube. The test is an indication of the functionality of the intrinsic and common pathways of the coagulation cascade, which is used by the body to form blood clots.
What does the result mean?
The result of aPTT testing provides information on the blood's ability to form clots. It helps identify deficiencies or abnormalities in clotting factors, assess the risk of bleeding disorders, and monitor the effectiveness of anticoagulant therapy.
Why conduct the analysis?
The test may be conducted:
- To investigate a bleeding or thrombotic episode.
- To assess the risk of excessive bleeding before a surgical procedure.
- To monitor anticoagulant therapy involving unfractionated heparin.
When to conduct the analysis?
The test may be conducted:
- When a patient is experiencing unexplained bleeding or thrombosis.
- When a patient is on unfractionated or intravenous (IV) heparin anticoagulant therapy.
In some cases, the test may be conducted during a pre-surgical screen.
What sample is required?
A blood sample is required. Blood should be drawn from a medium to large vein, typically in the arm, without the use of a tourniquet for an extended duration. It should then be collected into a sodium citrate blood tube to prevent activation of clot formation before testing.
Is any prior preparation necessary?
No prior preparation is necessary. However, patients should avoid eating a high fat meal before the sample is taken.
How is it performed?
The sample is sent to a laboratory, where it is analysed using a coagulation analyser, which measures the time it takes for blood to clot after the addition of specific reagents. The test is initiated by adding an activator to the blood sample, followed by calcium and a phospholipid source. The time taken for clot formation is then recorded.
What are the normal values?
The normal range for aPTT can vary depending on the laboratory and the specific reagents used for testing. However, typical reference ranges for aPTT are:
| aPTT (seconds) | Interpretation |
|---|---|
| 25 to 35 | Normal range for adults |
What does having altered values mean?
- Prolonged aPTT: Indicates deficiencies or abnormalities in clotting factors, such as factors VIII, IX, XI, or XII. It can also be seen in the presence of lupus anticoagulants, heparin therapy, or certain inherited clotting disorders.
- Shortened aPTT: Indicates elevated levels of clotting factors or in patients receiving certain types of therapy, such as high-dose oestrogen therapy.