Activated Partial Thromboplastin Time

The partial thromboplastin time (PTT) or activated partial thromboplastin time (aPTT or APTT) is a performance indicator measuring the efficacy of both the "intrinsic" (now referred to as the contact activation pathway) and the common coagulation pathways. Apart from detecting abnormalities in blood clotting,^[1] it is also used to monitor the treatment effects with heparin, a major anticoagulant. It is used in conjunction with the prothrombin time (PT) which measures the extrinsic pathway. Kaolin cephalin clotting time (KccT) is a historic name for the activated partial thromboplastin time^[2].

Method

A phlebotomist collects blood samples in vacu-tubes with oxalate or citrate to arrest coagulation by binding calcium. The specimen is then delivered to the laboratory. In order to activate the intrinsic pathway, phospholipid, an activator (such as silica, celite, kaolin, ellagic acid), and calcium (to reverse the anticoagulant effect of the oxalate) are mixed into the plasma sample . The time is measured until a thrombus (clot) forms. This testing is performed by a medical technologist.

The test is termed "partial" due to the absence of tissue factor from the reaction mixture.

Interpretation

Values below 25 seconds or over 39 s (depending on local normal ranges) are generally abnormal. Shortening of the PTT has little clinical relevance. Normal PTT times require the presence of the following coagulation factors: I, II, III, IV, V, VI, VIII, IX, X, XI, & XII. Notably, deficiencies in factors VII or XIII will not be detected with the PTT test. Prolonged APTT may indicate:

• use of heparin (or contamination of the sample)
• antiphospholipid antibody (especially lupus anticoagulant, which paradoxically increases propensity to thrombosis)
• coagulation factor deficiency (e.g. hemophilia)

To distinguish the above causes, mixing tests are performed, in which the patient's plasma is mixed (initially at a 50:50 dilution) with normal plasma. If the abnormality does not disappear, the sample is said to contain an "inhibitor" (either heparin, antiphospholipid antibodies or coagulation factor specific inhibitors), while if it does correct a factor deficiency is more likely. Deficiencies of factors VIII, IX, XI and XII and rarely von Willebrand factor (if causing a low factor VIII level) may lead to a prolonged aPTT correcting on mixing studies.

Comparison of coagulation serology^[3]

view • talk • edit Condition	Prothrombin time (PT)	Partial thromboplastin time (aPTT)	Bleeding time (BT)	Platelet count
Uncomplicated bleeding	unaffected	unaffected	unaffected	unaffected
Thrombocytopenia	unaffected	unaffected	prolonged	decreased
Early Liver failure	prolonged	unaffected	unaffected	unaffected
End-stage Liver failure	prolonged	prolonged	prolonged	decreased
Uremia	unaffected	unaffected	prolonged	unaffected
Congenital afibrinogenemia	prolonged	prolonged	prolonged	unaffected
Factor V deficiency	prolonged	prolonged	unaffected	unaffected
Factor X deficiency as seen in amyloid purpura	prolonged	prolonged	unaffected	unaffected
Von Willebrand disease	unaffected	prolonged	prolonged	unaffected
Haemophilia	unaffected	prolonged	unaffected	unaffected
Glanzmann's thrombasthenia	unaffected	unaffected	prolonged	unaffected
Bernard-Soulier syndrome	unaffected	unaffected	prolonged	decreased
Disseminated intravascular coagulation	prolonged	prolonged	prolonged	decreased
Thrombotic thrombocytopenic purpura	unaffected	unaffected	prolonged	decreased
Vitamin K deficiency or Warfarin	prolonged	prolonged	unaffected	unaffected
Aspirin	unaffected	unaffected	prolonged	unaffected
Thrombolytic drugs such as tissue plasminogen activator and streptokinase	prolonged	prolonged	prolonged	unaffected

 History

The aPTT was first described in 1953 by researchers at the University of North Carolina at Chapel Hill.^[4]

See also

• Prothrombin time (PT)

References

1. ^ "MedlinePlus Medical Encyclopedia: Partial thromboplastin time (PTT)". http://www.nlm.nih.gov/medlineplus/ency/article/003653.htm. Retrieved 2009-01-01. 
2. ^ "KCCT - General Practice Notebook". GP Notebook. Oxbridge Solutions Ltd. http://www.gpnotebook.com/simplepage.cfm?ID=-1207566306. Retrieved 2010-06-08. 
3. ^ Goljan, Edward (2010). Edward Goljan. ed (in English). Rapid Review Pathology (3rd ed.). Philadelphia, PA, USA: Mosby. ISBN 978-0-323-06862-8. 
4. ^ Langdell RD, Wagner RH, Brinkhous KM (1953). "Effect of antihemophilic factor on one-stage clotting tests; a presumptive test for hemophilia and a simple one-stage antihemophilic factor assy procedure". J. Lab. Clin. Med. 41 (4): 637–47. PMID 13045017.