|APTT: Activated Partial Thromboplastin Time|
The partial thromboplastin time (PTT) was developed in 1953 after experiments with various thromboplastins for the prothrombin time yielded a test that was sensitive to hemophilia. It was observed that thromboplastin derived from platelets required an additional factor(s) to initate clotting-thus the name, partial thromboplastin as opposed to "complete" thromboplastins. A more modern modification employs an activator for the contact factors giving us the activated PTT (aPTT).
The aPTT measures clotting through the intrinsic pathway. Reagents (activator, platelet phospholipid and calcium) are added to test plasma and the time to clot formation is measured. The aPTT reflects the functional quantity of prekallekrien, high molecular weight kininogen, factors XII, XI, IX, VIII, X, V, II (prothrombin) and I (fibrinogen). The test is also sensitive to clot inhibitors such as heparin. The aPTT is a global test that is sensitive to many conditions,which relate to the intrinsic and common pathways.
Because of the global nature, the aPTT is not specific. A single factor deficiency (listed above) below an approximate 30% level will give a prolonged test. Multiple factor deficiencies at a moderate level may also cause prolongation. Other causes of prolongation include, specific and nonspecific antibodies, interfering proteins, FDP's and anticoagulants (e.g. heparin). The aPTT has variable sensitivity to Vitamin K deficiencies (multiple factor II, VII, IX and X deficiency) and to the effect of oral anticoagulants. Heparin, in therapeutic doses, will prolong the aPTT. The test has variable sensitivity to "lupus anticoagulants". The aPTT is subject to numerous specimen-collection artifacts, which unfortunately are the most common causes of prolongation.
A normal aPTT means that intrinsic and common pathway clotting factors are present in at least 30% activity. A prolonged test often represents the beginning of an investigation, not an end to itself.
Reference range: Currently 24.0- 35.0 seconds (the range varies slightly by lot #)
All unexplained prolonged results are potentially significant since the sensitivity is low. Given the complex nature of hemostasis, the test can never be used alone to rule out a bleeding disorder that is indicated by clinical bleeding or by a positive history.
Note: All coagulation tests are sensitive to collection criteria , which cause prolongation.
Blood to anticoagulant ratio: This must be 9:1 in the collection tube. Specimens with inadequate blood or patients with hematocrits exceeding 55% are likely to have falsely prolonged results. This is why many specimens are rejected as quantity not sufficient (QNS).
Collection of specimens through heparinized lines.
Clotted specimens: Factors (mostly fibrinogen) are consumed in the clotting process.
Common diseases to consider if the aPTT is abnormal:
1. Acquired disorders:
Vitamin K deficiency
Advanced Liver disease
Disseminated Intravascular Coagulation (FDP/ D-dimer)
Myeloma, Waldenstrom's, etc.
Acquired specific antibody (e.g. to FVIII)
© 2004 Medical Center of Louisiana