Prothrombin time: PT
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The prothrombin time (PT) was developed in the 1930s to test a theoretical model that consisted of four coagulation factors. The test was known to be sensitive to prothrombin (factor II), hence the name.

The prothrombin time measures clotting through the extrinsic pathway. A reagent source of tissue factor and calcium is added to test plasma. The prothrombin time reflects the functional quantity of factors VII, X, V, II (prothrombin) and I (fibrinogen). The test is also moderately sensitive to clot inhibitors such as heparin. The PT, as well as the aPTT, is a global test that is sensitive to many conditions, which relate to the extrinsic pathway.

Because of the global nature, the PT is not specific. A single factor deficiency (from the list 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, FDPs and anticoagulants (e.g. heparin ). The PT is very sensitive to Vitamin K deficiencies (II, VII, IX and X or multiples) and to the effect of oral anticoagulants. Heparin , in high doses will prolong the PT but heparin has much more effect on the aPTT. The test has variable sensitivity to "lupus anticoagulants". Last but not least, the test is subject to numerous specimen-collection artifacts, which unfortunately are the most common causes of prolongation.

A normal prothrombin time means that extrinsic 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.

Interpreting results:
Reference range: Currently 10.0- 13.0 seconds (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.

False positives:
Note: All coagulation tests are sensitive to collection criteria , which cause prolongation.

1. 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).
2. Collection of specimens through heparinized lines.
3. Clotted specimens: Factors (mostly fibrinogen) are consumed in the clotting process.

Common diseases to consider if the PT is abnormal:

1. Acquired disorders:
Vitamin K deficiency Warfarin therapy*
Advanced Liver disease DIC
"Lupus anticoagulant" Myeloma, WaldenstromÕs, etc.
*beware of attempted murder or suicide with rat poison.
2. Inherited disorders:
Inherited Factor deficiency VII, X, V, II, (these are very rare)
Dysfibrinogenemia

The bottom line:
This test is widely used to screen for hemostatic defects, although numerous reports indicate that it is not a useful screening test. The PT is useful to monitor oral anticoagulant therapy especially when the INR is utilized. Abnormal results from screening are investigated with factor assays, mixing studies and / or correlation with other clinical findings.

 

© 2004 Medical Center of Louisiana
Page updated: 9/24/04