Partial thromboplastin time
| Partial thromboplastin time |
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
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 (tissue factor pathway) coagulation pathways. Apart from detecting abnormalities in blood clotting, it is also used to monitor the treatment effects with heparin, a major anticoagulant.
| Reference Range |
| Around 35-55 seconds |
| Under heparin therapy according to indication: 1.5-2.5x longer |
| PTT is not prolonged under therapy with low-molecular heparins |
Differential Diagnosis
In alphabetical order: [1] [2]
Prolonged
- Clotting factor deficiency
- DIC
- Drugs
- Drug side effect: Thrombin human
- Fibrinolytic states
- Fibrinogen deficiency
- Heparin
- Hypoalbuminemia
- Impaired fibrin polymerization
- Liver Disease
- Subcutaneous heparin injection
- Therapy with unfractionated heparin
- Thrombolytics
- Vitamin K deficiency
- Von Willenbrand's Disease
- Warfarin
Methodology
Blood is collected, by a phlebotomist, with oxalate or citrate which arrest coagulation by binding calcium. This specimen is 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. 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 studies 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.
History
The aPTT was first described in 1953.[3]
See also
- Prothrombin time (PT)
References
- ↑ Sailer, Christian, Wasner, Susanne. Differential Diagnosis Pocket. Hermosa Beach, CA: Borm Bruckmeir Publishing LLC, 2002:77 ISBN 1591032016
- ↑ Kahan, Scott, Smith, Ellen G. In A Page: Signs and Symptoms. Malden, Massachusetts: Blackwell Publishing, 2004:68 ISBN 140510368X
- ↑ 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.
cs:Aktivovaný parciální tromboplastinový čas
de:Partial Thromboplastin Time
it:Tempo di Tromboplastina
nl:Geactiveerde partiële tromboplastinetijd