Activated Partial Thromboplastin Time (aPTT)

Definition - What does Activated Partial Thromboplastin Time (aPTT) mean?

Activated partial thromboplastin time (aPTT) is a blood test used to determine the time it takes for the blood to clot. The presence of clotting factors in the blood account for its natural tendency to clot. The normal reference range of the aPTT is 30-40 seconds. Any condition that interferes with the clotting of blood will prolong the aPTT signifying that the blood takes longer than usual to clot. One of the conditions that prolong the aPTT is the antiphospholipid syndrome, which puts a woman at significant risk of recurrent pregnancy loss (RPL).

Although conditions with clotting factor deficiencies usually cause a rise in aPTT, in pregnancy, a woman's blood has an inherent tendency to clot due to the lack of clotting factor inhibitors. This shortens the aPTT, which can also cause RPL. Testing the aPTT levels is, therefore, an integral component in evaluating women with a history of recurrent pregnancy loss.

FertilitySmarts explains Activated Partial Thromboplastin Time (aPTT)

To test aPTT, a technician draws the blood from a vein in the arm and collects it in a test tube containing a blood thinner. Some quantity of the plasma is then transferred from this test tube into a measuring test tube. To reverse the effect of the blood thinner, calcium is added into the measuring tube. The final step is the addition of an activator (to activate the intrinsic pathway of coagulation) into the test tube and measuring the time the sample takes to clot.

Some of the conditions that prolong aPTT include:

  • Deficiency of clotting factors: Usually associated with bleeding (like in hemophilia) but can also result in the formation of blood clots in veins (such as in factor XII deficiency). In pregnancy, factor XII deficiency and the resultant enhanced tendency of the blood to clot can cut off the blood supply to the uterus leading to a miscarriage. Factor XII deficiency is also one of the underlying causes of recurrent IVF failure.
  • Liver disease
  • Heparin therapy
  • Antiphospholipid syndrome (APS): A condition in which autoantibodies are directed against the charged structures on the surface of the cells known as phospholipids. APS causes the formation of blood clots in both arteries and veins as well as pregnancy-related complications like miscarriages, preterm delivery, stillbirth, and preeclampsia. Although not associated with bleeding, APS causes a paradoxical rise in aPTT.

A prolonged aPTT usually reflects clotting factor deficiencies whereas a shortened aPTT reflects high levels of clotting factors in the blood. Conditions associated with a shortened aPTT (such as pregnancy), could, therefore, also contribute to blood clots, which slow the blood flow to the uterus and placenta causing miscarriages.

This is why doctors usually order an aPTT in women with a history of frequent miscarriages to determine the underlying cause.

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