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Activated Partial Thromboplastin Time APTT

Also known as

PTT Test/PT with INR Test, APTT, and aPTT, Partial Thromboplastin Time, PTT

This test is ordered by the doctor when he or she believes there is an issue with one or more clotting factors. The doctor orders this test to investigate a possible bleeding disorder, a thrombotic episode which is also known as a blood clot, recurrent miscarriages, or diagnoses the antiphospholipid syndrome, also known as APS. It is also ordered to look into unfractionated heparin anticoagulant therapy. Sometimes it is ordered as part of an evaluation before surgery or other kinds of invasive procedures.
The test is ordered by the doctor when the patient has unexplained bleeding, recurrent miscarriages, or inappropriate blood clotting. It is also ordered before a scheduled surgery or when the patient has been taking unfractionated standard heparin anticoagulants.
This is a blood test. The healthcare practitioner draws blood using a syringe from a vein in the arm.
There is no special preparation needed. But the patient should avoid eating a meal high in fat prior to the blood draw as it may interfere with the test and produce inaccurate results.
The Activated Partial Thromboplastin Time is a screening test. The test allows the doctor to evaluate whether the patient’s body is able to form blood clots appropriately. The test measures how many seconds it takes for a blood clot to form in the blood when reagents are added to the sample. The PTT evaluates the amount and the function of the protein in the blood. These are called clotting or coagulation factors and are an important component of blood clot formation. When someone is hurt, their tissues or blood vessel walls take the impact and get injured, which leads to bleeding. Once the bleeding starts, hemostasis begins in the blood. This involves platelets sticking together and clumping at the site of injury. Simultaneously the coagulation cascade is set into motion, and coagulation factors are triggered in a step-by-step process. The cascade chain creates reactions that allow threads called fibrin to form and mesh into a net that sticks to the injury site. The fibrin threads stabilize it, forming a stable blood clot that seals off the injury to the blood vessels. The process also halts additional loss of blood and allows the damaged area to heal. The process of hemostasis is essential for the healthy functioning of the body. Each part of the process must work flawlessly and be present in adequate amounts in order to allow the blood clot to form. If one or two of the factors is too low, or if the factors fail to do their job properly, then the blood clot formed may not be stable or complete, leading to excessive bleeding.
The result of the tests is used to compare to a reference range or the normal reference interval for clotting time. If the blood clot takes longer to form in the sample, the PTT is declared to be prolonged. When the aPTT is ordered by the doctor to assess bleeding or clotting episodes or to eliminate the possibility of a clotting disease, the test is ordered alongside a related test called the prothrombin time. The results of both the tests are used in conjunction to deliver any kind of verdict on a possible bleeding or clotting disorder. In addition, 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). On the other hand, the PT test assesses the coagulation factors VII, X, V, II, and I (fibrinogen).
Your doctor may order an aPTT with other tests such as a PT. These are ordered when you have:
  1. Unexplained bleeding or easy bruising
  2. A blood clot in a vein or artery
  3. An acute condition such as disseminated intravascular coagulation (DIC) may cause both bleeding and clotting as coagulation factors are used up at a rapid rate
  4. A chronic condition such as liver disease that may affect clotting
  5. The doctor orders an aPTT when
  6. As part of an investigation into lupus anticoagulant, anticardiolipin antibodies, and antiphospholipid syndrome
  7. The patient has had a blood clot
  8. A woman has had recurrent miscarriages
  9. The patient has switched from heparin therapy to longer-term warfarin (Coumadin®) therapy, the two are overlapped, and both the PTT and PT are monitored until the patient has stabilized.
  10. The patient has a scheduled surgical operation. He or she may have a PTT prior to surgery when the surgery carries an increased risk of blood loss.
  11. If the patient has a clinical history of bleeding, such as frequent or excessive nose bleeds and easy bruising, which may indicate the presence of a bleeding disorder.
The results of the test are usually reported in seconds. An aPTT result that falls within the reference interval usually indicates normal clotting function. But it must be noted that even when the aPTT result is normal, there is a possibility of mild to moderate deficiencies of a single coagulation factor. The aPTT may not be prolonged until the factor levels have decreased to 30% to 40% of normal. On the other hand, lupus anticoagulants can be present but not prolong the PTT result. If the lupus anticoagulant (LA) is suspected, a more sensitive LA-sensitive APTT or a dilute Russell viper venom time (DRVVT) can be used to test for it. A prolonged PTT means that clotting is taking longer to occur than normal and may be due to a variety of causes: 1. Underlying conditions that lead to lower levels of clotting factors:
  1. Liver disease—the liver produces the most amount of coagulation factors. Therefore, liver disease may lead to prolonged PT and PTT. However, PT is more likely to be prolonged than PTT.
  2. Vitamin K deficiency—Vitamin K is crucial for the formation of several clotting factors. While Vitamin K deficiencies are not common, they can occur due to an extremely poor diet, malabsorption disorders, or prolonged use of certain antibiotics, for example. PT is more likely to be prolonged than is PTT.
2. Less commonly inherited clotting factor deficiencies:
  1. von Willebrand disease (vWD) is the most common inherited bleeding disorder. The disease impacts platelet function due to decreased von Willebrand factor. PTT is normal in most cases of vWD but can be prolonged in severe vWD.
  2. Hemophilia A and hemophilia B (Christmas disease) are two other inherited bleeding disorders resulting from a decrease in factors VIII and IX, respectively.
  3. Deficiencies of other coagulation factors, like factors XII and XI. Deficiency in XI can cause abnormal bleeding, but deficiency of XII is not associated with bleeding risk in the body.
3. Inhibitors that impact results:
  1. Lupus anticoagulant, a non-specific inhibitor, is an autoantibody (antiphospholipid antibody) that interferes with the aPTT by targeting substances called phospholipids that are used in the aPTT. Lupus anticoagulants can prolong the aPTT result.
However, they are more commonly associated with excessive clotting in the body. A person who produces these antibodies may be at an increased risk for a blood clot.
  1. Some specific inhibitors may impact results. These are relatively rare. Specific inhibitors are antibodies that specifically target certain coagulation factors (known as factor-specific inhibitors), affecting how they function, such as antibodies that target factor VIII. Factor-specific inhibitors can lead to heavy bleeding.
  2. Heparin—is an anticoagulant and will prolong an aPTT either as part of a sample or treatment.
  3. Warfarin anticoagulation therapy may prolong the aPTT. The prothrombin time/international normalized ratio is used to monitor warfarin therapy.
Related Tests

Prothrombin Time and International Normalized Ratio (PT/INR), Fibrinogen, D-dimer, Thrombin Time, Lupus Anticoagulant Testing, Activated Clotting Time (ACT), Coagulation Factors, Platelet Count, Heparin Anti-Xa, von Willebrand Factor, Antiphospholipid Antibodies