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 Hemostasis Testing Options |
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Laboratory Tests
Blood tests have been developed to help clinicians
identify patients with bleeding disorders. Two tests
that are commonly requested include Prothrombin
Time (PT) and Activated Partial Thromboplastin Time
(aPTT).
Prothrombin Time
Prothrombin time (PT) involves the addition of thromboplastin
and calcium to citrated plasma. Clot formation occurs
in 11 to 13 seconds via the extrinsic pathway. The
test is sensitive to deficits in Factors VII, X,
V, II, fibrinogen, and circulating inhibitors. It
is widely recognized that thromboplastin preparations
differ in their ability to activate coagulation,
with a strong activator resulting in a normal PT
even with some degree of deficiency in clotting
factors.1
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Since the PT is focused on the second phase of hemostasis
(the extrinsic pathway),1,2
and because the risk of bleeding from platelet disorders
is not reflected by alterations in PT and aPTT,3
its value is limited to screening for platelet disorders
of primary hemostasis.1,2
Activated Partial Thromboplastin Time
Activated Partial Thromboplastin Time (aPTT) was
originally developed to measure clotting times with
the addition of phospholipids and calcium to citrated
plasma. However, sensitivity and reproducibility
were improved by initiating coagulation with a particulate
activator of the intrinsic pathway and, as a result,
has been adopted by almost all of the laboratories
performing coagulation studies. There are two distinct
benefits to using this method:1
- Sensitivities to heparin and Factors
VIII and IX deficiencies
- Short incubation period
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aPTT is focused on the second phase of hemostasis
(the intrinsic pathway) and is, therefore, of
limited value in screening for platelet disorders
of primary hemostasis.1,2
For information on other tests for platelet dysfunction,
click on the following:
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