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As part of an investigation of a possible bleeding disorder or blood clot (thrombotic episode); to help investigate recurrent miscarriages or diagnose antiphospholipid syndrome (APS); as needed to monitor unfractionated (standard) heparin anticoagulant therapy; as indicated as part of an evaluation before surgery or other invasive procedure
When you have unexplained bleeding, inappropriate blood clotting, or recurrent miscarriages; sometimes when you are on standard heparin anticoagulant therapy; sometimes before a scheduled surgery
A blood sample drawn by needle from a vein in your arm
None; however, a high-fat meal prior to the blood draw may interfere with the test and should be avoided.
The partial thromboplastin time (PTT; also known as activated partial thromboplastin time (aPTT)) 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 sample of blood after substances (reagents) are added. The PTT assesses the amount and the function of certain proteins in the blood called coagulation or clotting 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 stick 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 in a step-by-step process. Through the cascading reactions, threads called fibrin form and crosslink into a net that clings to the injury site and stabilizes it. 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 part of this hemostatic process must function properly and be present in sufficient quantity for normal blood clot formation. If the amount of one or more factors is too low, or if the factors cannot do their job properly, then a stable clot may not form and bleeding continues.
With a PTT, your result is compared to a normal reference interval for clotting time. When your PTT takes longer than normal to clot, the PTT is considered "prolonged."
When a PTT is used to investigate bleeding or clotting episodes or to rule out a bleeding or clotting disease (e.g., preoperative evaluation), it is often ordered along with a prothrombin time (PT). A healthcare practitioner will evaluate the results of both tests to help rule out or determine the cause of bleeding or clotting disorder.
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.
The PTT is used primarily to investigate unexplained bleeding or clotting. It may be ordered along with a prothrombin time (PT/INR) to evaluate 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.
By evaluating the results of the two tests together, a healthcare 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:
The PTT may be ordered along with other tests such as a PT when you have:
A PTT may be ordered:
PTT results are typically reported in seconds.
A PTT result that falls within a laboratory's reference interval usually indicates normal clotting function. However, even with a normal PTT result, 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. On the other hand, 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. (See below for more about LA-sensitive PTT.)
A prolonged PTT means that clotting is taking longer to occur than normal and may be due to a variety of causes.
A prolonged PTT may be due to:
Results of the PTT are often interpreted with results of the PT in determining what condition may be present.
PT result | ptt result | Common condition present |
---|---|---|
Prolonged | Normal | Liver disease, vitamin K deficiency, decreased or defective factor VII, chronic DIC, warfarin or other vitamin K antagonist (e.g., brodifacoum in some cannabinoids) |
Normal | Prolonged | Hemophilia A or B (decreased or defective factor VIII or IX) or factor XI deficiency, von Willebrand disease (severe form), factor XII deficiency, or lupus anticoagulant present |
Prolonged | Prolonged | Decreased or defective factor I (fibrinogen), II (prothrombin), V or X, severe liver disease, acute 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. |
A shortened PTT may be due to:
The LA-sensitive PTT (LA-PTT or PTT-LA) is a variant PTT, designed to evaluate the presence of lupus anticoagulant (LA), an antibody associated with clotting episodes and recurrent miscarriages. The LA-PTT uses a low phospholipid reagent that is optimized for detecting lupus anticoagulants and is therefore more sensitive to LA. 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.
Examples of other testing that may be done along with a PTT or in follow up to abnormal results include:
In a few situations, it is not.
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 should be closely monitored and may be treated with frequent replacement infusions of the missing clotting factor.
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:
Sources Used in Current Review
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(June 2013) Elevated APTT? How to Best Follow Up. Journal of Family Practice. Available online at https://www.mdedge.com/jfponline/article/76289/cardiology/elevated-aptt-how-best-follow. Accessed March 2019.
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Activated Partial Thromboplastin Time (APTT). University of Rochester Medical Center. Available online at https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid=aptt. Accessed March 2019.
Vandiver JW, Vondracek T. Antifactor Xa levels versus activated partial thromboplastin time for monitoring unfractionated heparin. Pharmacotherapy. 2012 Jun;32(6):546-58. Available online at https://www.ncbi.nlm.nih.gov/pubmed/22531940. Accessed March 2019.
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