Difference Between aPTT and PTT

Main Difference – aPTT vs PTT

Thromboplastin is a plasma protein that aids blood coagulation by catalyzing the conversion of prothrombin to thrombin. It is also known as the coagulation factor III, a tissue factor. It activates the extrinsic pathway during blood clotting. In the lab, a derivative of thromboplastin known as partial thromboplastin is produced to measure the intrinsic pathway. Partial thromboplastin is a phospholipid. aPTT (activated partial thromboplastin time) and PTT (partial thromboplastin time) are two types of medical tests used to characterize blood coagulation in intrinsic pathway. The main difference between aPTT and PTT is that aPTT uses an activator to reduce the time taken for blood clotting whereas PTT operates under regular conditions for blood clotting.

Key Areas Covered

1. What is aPTT
– Definition, Facts, Mechanism
2. What is PTT
– Definition, Facts, Mechanism
3. What are the Similarities Between aPTT and PTT
– Outline of Common Features
4. What is the Difference Between aPTT and PTT
– Comparison of Key Differences

Key Terms: aPTT (Activated Partial Thromboplastin Time), Blood Coagulation, Intrinsic Pathway, Heparin, Partial Thromboplastin, PTT (Partial Thromboplastin Time), Thromboplastin

What is aPTT

aPTT (activated partial thromboplastin time) refers to a test of blood coagulation used to evaluate the clotting factors of the intrinsic pathway. The main purpose of aPTT is to screen bleeding tendencies and to monitor heparin therapy. Blood clotting is a multi-protein cascade governed by proteins called clotting factors. Clotting factors are designated by Roman numbers. Heparin is an anticlotting drug given to patients. It inhibits factor X and thrombin but, activates anti-thrombin. The deficiencies of the coagulation factors such as Factors V, VIII, IX, X, XI, and XII increases the aPTT. Hodgkin lymphoma, DIC, hypofibrinogenemia, cirrhosis, leukaemia, vitamin K deficiency, von Willebrand’s disease and other drug therapies also increase aPTT level. The intrinsic pathway of blood coagulation is shown in figure 1.

Figure 1: Intrinsic Pathway

Decalcified blood is used for the aPTT test. Then the blood plasma is separated by centrifugation. Ionized calcium and activating substances are added to the blood plasma, initiating the intrinsic pathway. Kaolin and cephalin are two types of substances added to the blood plasma. Kaolin or hydrated aluminium silicate serves as the activator of the contact-dependent factor XII while cephalin serves as a platelet phospholipids. The time taken for the formation of the clot, which is measured in seconds, is known as partial thromboplastin time. The normal aPTT value is 35 seconds.

What is PTT

PTT (partial thromboplastin time) refers to a test used to measure the time taken for the blood clotting. This is used for the diagnosis of bleeding problems. During PTT, the integrity of the intrinsic system is measured by means of factors VIII, IX, XI, and XII. PTT also evaluates the common pathway. Generally, both intrinsic and extrinsic pathway activate the common pathway by clotting factor X. Common pathway is involved in the formation of fibrin from fibrinogen. Fibrin serves as a sieve that collects platelets to form a blood clot. A blue-top vacutainer tube used for the collection of blood for PTT is shown in figure 2.

Figure 2: Blue-Top Vacutainer

The procedure of the test is same as aPTT steps, but an activator is not used in PTT. Therefore, the time taken for the test is longer than that of the aPTT. The increased levels of PTT indicate a missing or defective clotting factor. Further diagnosis of the defective clotting factors requires other sensitive assays. Liver diseases also increase the production of clotting factors, increasing the levels of PTT.

Similarities Between aPTT and PTT

aPTT and PTT are two medical tests used to characterize blood coagulation.
Both aPTT and PTT use partial prothrombin, a phospholipid.
Both aPTT and PTT measure the intrinsic pathway of blood coagulation.
Blood plasma is used in both tests.
Both aPTT and PTT is useful in the diagnosis of bleeding problems and blood coagulation disorders.

Definition

aPTT: aPTT (activated partial thromboplastin time) refers to a test of activated blood coagulation used to evaluate the clotting factors of the intrinsic pathway.

PTT: PTT (partial thromboplastin time) refers to a test used to measure the time taken for the blood clotting in order to diagnose bleeding problems.

Significance

aPTT: An activator is used in aPTT.

PTT: PTT does not use an activator.

Reference Range

aPTT: The reference range of aPTT is 30-40seconds.

PTT: The reference range of PTT is 60-70 seconds.

Narrowing the Reference Range

aPTT: The reference range of aPTT is narrowed by adding an activator.

PTT: The reference range is the regular time of blood clotting in PTT.

Critical Values

aPTT: More than 70 seconds in aPTT test signifies spontaneous bleeding.

PTT: More than 100 seconds in PTT test signifies spontaneous bleeding.

Type of Clotting Factors

aPTT: aPTT measures factors such as V, VIII, IX, X, XI, and XII.

PTT: PTT measures factors such as VIII, IX, X, and XII.

Sensitivity to Heparin

aPTT: aPTT is more sensitive to heparin.

PTT: PTT is less sensitive to heparin.

Role

aPTT: aPTT evaluates bleeding disorders and heparin therapy.

PTT: PTT evaluates the intrinsic pathway as well as the common pathway.

Conclusion

aPTT and PTT are two types of tests used to measure the blood coagulation rates of the intrinsic pathway. Partial thromboplastin is the substance used for this purpose. aPTT uses an activator to narrow the reference range but, PTT does not use an activator. Therefore, the main difference between APTT and PTT is the usage of an activator.

Reference:

1. Blood Laboratory: Hemostasis: PT and PTT tests, Available here.

Image Courtesy:

1. “Coagulation in vivo” By Dr Graham Beards – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Blue Top” By Gene Hobbs – Own work (CC BY-SA 4.0) via Commons Wikimedia