What is Partial Thromboplastin Time (PTT) / APTT?

A partial thromboplastin time (PTT) test measures the time it takes for a blood clot to form. Normally, when you get a cut or injury that causes bleeding, proteins in your blood called coagulation factors work together to form a blood clot. The clot stops you from losing too much blood.

You have several coagulation factors in your blood. If any factors are missing or defective, it can take longer than normal for blood to clot. In some cases, this causes heavy, uncontrolled bleeding. A PTT test checks the function of specific coagulation factors.

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.

• 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).

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/INR) to evaluate the process that the body uses to form blood clots to help stop bleeding. These tests are usually the starting point 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:

• 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 against a coagulation factor (known as a factor-specific inhibitor) is present in the blood.


• To detect nonspecific autoantibodies (antiphospholipid antibodies), such as lupus anticoagulants, 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; however, some labs now use the anti-Xa test to monitor heparin therapy. Heparin is an anticoagulation drug that is given intravenously (IV) or by injection to prevent and 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 are sometimes selectively performed before a scheduled surgery or other invasive procedures to screen for potential bleeding tendencies.