Activated Partial Thromboplastin Time (aPTT)

Complications associated with the coagulation cascade are serious medical conditions in maternity care. The Activated Partial Thromboplastin Time may be a useful tool for prediction, prevention, and management of bleeding complications. 

Activated Partial Thromboplastin Time Testing

The Activated Partial Thromboplastin Time (aPTT) test is used to assess clotting ability. It measures the time it takes for blood to clot after certain clotting factors are activated with reagents. 

The coagulation cascade is a series of interconnected enzymatic reactions that culminate to form a blood clot. The cascade is divided into three main pathways: the extrinsic pathway, intrinsic pathway, and common pathway. 

The intrinsic pathway is trigged when exposed collagen activates Factor XII. Factor XIIa activates Factor XI, which then activates Factor IX in the presence of Factor VIII and calcium. Then, Factor IXa, Factor VIIIa, and calcium form a complex to active Factor X to Factor Xa, initiation the common pathway. 

• Factor VIII: Also known as antihemophilic factor A. Factor VIII is needed for the formation of tenase complex, which activates factor X. Deficiencies lead to hemophilia A.
• Factor IX: Also known as Christmas factor. Factor IX is a component of tenase complex, along with factor VIII and calcium. It activates factor X. Deficiencies lead to hemophilia B.
• Factor XI: Also known as plasma thromboplastin antecedent (PTA). Factor XI is part of the amplification of the coagulation cascade. It activates factor IX. Deficiencies lead to hemophilia C.
• Factor XII: Also known as Hageman factor. Factor XII is activated on by foreign substances. Factor XII activates factor XI.
• Prekallikrein: Also known as Fletcher factor. Prekallikrein is activated on by foreign substances leading to the activation of factor XII.
• High Molecular Weight Kininogen: Cofactor of Prekallikrein.

The aPTT test assesses the intrinsic pathway. Partial Thromboplastin Time (PTT) and aPTT are similar and are often used interchangeably but vary slightly based on the activator.

​Activated Partial Thromboplastin Time in Pregnancy

During pregnancy there is an increase in the production of clotting factors. Factor I, Factor II, Factor VIII, Factor IX, and Factor XIII production increases, while protein S and protein C activity decreases. In addition, there is an increase in the thrombin-antithrombin complexes. These changes help to balance maternal clotting ability. 

The aPTT, and PTT ranges decrease slightly in pregnancy with the increased hypercoagulative state of pregnancy. There is a measurable decrease in aPTT between the first and third trimester in preparation for childbirth. In cases of severe hemorrhage in childbirth there is a sudden drop in fibrinogen and prolonged aPTT soon after onset.

Prolonged Activated Partial Thromboplastin Time in Pregnancy

There is limited data on the implications of prolonged aPTT in pregnancy.

Hypothyroidism has been shown to have a pronounced effect on aPTT testing due to its association with the expression and regulation of blood clotting factors. There is also a connection between subclinical hypothyroidism in the third trimester and increased risk of postpartum hemorrhage. 

Causes of Prolonged Activated Partial Thromboplastin Time in Pregnancy

• Lupus Anticoagulant Syndrome
• Antiphospholipid Antibody Syndrome
• Hypothyroidism
• Von Willebrand Disease
• Genetic Deficiencies in Clotting Factors
• Vitamin K Deficiency
• Liver Disease
• Rheumatoid Arthritis

Shortened Activated Partial Thromboplastin Time in Pregnancy

There is limited data on the implications of shortened aPTT in pregnancy.

Causes of Shortened Activated Partial Thromboplastin Time in Pregnancy

• Hyperthyroidism
• Lab Error
• Acute Internal Bleeding
• Cancer

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The content in this article is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding your medical condition. ​

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