Partial thromboplastin time

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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:

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.

Condition Prothrombin time Partial thromboplastin time Bleeding time Platelet count
Vitamin K deficiency or Warfarin prolonged prolonged unaffected unaffected
Disseminated intravascular coagulation prolonged prolonged prolonged decreased
Von Willebrand disease unaffected prolonged prolonged unaffected
Haemophilia unaffected prolonged unaffected unaffected
Aspirin unaffected unaffected prolonged 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
Glanzmann's thrombasthenia unaffected unaffected prolonged unaffected
Bernard-Soulier syndrome unaffected unaffected prolonged decreased

History

The aPTT was first described in 1953 by researchers at the University of North Carolina at Chapel Hill.[3]

See also

References

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cs:Aktivovaný parciální tromboplastinový čas

de:Partial Thromboplastin Time es:Tiempo de Tromboplastina Parcial Activado fr:Temps de céphaline activée it:Tempo di Tromboplastina nl:Geactiveerde partiële tromboplastinetijd pl:APTT

pt:Tempo de tromboplastina parcial ativada
  1. "MedlinePlus Medical Encyclopedia: Partial thromboplastin time (PTT)". Retrieved 2009-01-01. 
  2. "KCCT - General Practice Notebook". GP Notebook. Oxbridge Solutions Ltd. Retrieved 2010-06-08. 
  3. 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.