P-APT time (APTT), Activated Partial Thromboplastin Time, is a functional test that measures the activity of the intrinsic coagulation system. The test covers several coagulation factors, primarily factors VIII, IX, XI, and XII, as well as fibrinogen. These factors are activated in a cascade-like process that results in the formation of fibrin – a protein that stabilizes the blood clot and enables effective hemostasis.
APTT is a time measurement (in seconds) of how long it takes plasma to coagulate after coagulation has been activated with specific reagents. Since reagents and methodologies can differ between laboratories, reference ranges may vary, but APTT is normally between 20–30 seconds. A prolonged APTT means that the blood clots more slowly than normal and is often used to monitor treatment with heparin or to identify coagulation factor deficiencies.
Functional role
P-APT time (APTT) reflects the activity of coagulation factors in the intrinsic system and is therefore a central test in the investigation of hemostasis.
- Coagulation regulation: APTT measures the function of several factors necessary to initiate and amplify the coagulation cascade. Deficiency or impaired function of these factors prolongs APTT.
- Liver synthesis: Since many coagulation factors are produced in the liver, APTT may be prolonged in liver failure.
- Autoimmune influence: The presence of antibodies, for example in antiphospholipid syndrome, can interfere with coagulation and prolong APTT.
- Bleeding risk and anticoagulants: APTT is used to monitor heparin treatment and to assess the risk of bleeding or thrombosis.
Use of APTT
P-APT time (APTT) is analyzed in several clinical situations where coagulation ability needs to be investigated or monitored.
Prolonged APTT
A prolonged APTT indicates slower coagulation and may be seen in:
- Heparin treatment – used for dosage adjustment and monitoring.
- Coagulation factor deficiencies – for example in hemophilia A (factor VIII deficiency) or hemophilia B (factor IX deficiency).
- Von Willebrand disease – affects factor VIII and often leads to prolonged APTT.
- Liver disease or vitamin K deficiency – impaired synthesis of coagulation factors.
- Autoimmune conditions – e.g. antiphospholipid syndrome with antibodies against coagulation factors.
Normal or shortened APTT
Normal APTT values are seen in healthy individuals. Shortened APTT is rare but may indicate increased coagulation activity, for example in inflammation, pregnancy, or thrombotic conditions.
Use cases include:
- Dosage adjustment during heparin therapy
- Preoperative assessment of bleeding risk
- Investigation of suspected bleeding disorders
- Evaluation of liver function
Related blood markers
When assessing coagulation disorders, APTT is often analyzed together with:
- Fibrinogen: determines clot stability.
- Liver function tests: ALT, AST, bilirubin, and albumin provide complementary information.
- Platelets: essential for primary hemostasis and bleeding control.