Prothrombin Time (PT) Test, Principle, Normal and “High” Values

Hemostasis is the mechanism by which the body stops blood loss. It involves a series of steps first to rapidly plug the leak and then to secure it followed by permanent healing processes that restores the integrity of the blood vessel. One of the important phases of hemostasis is the coagulation phase which involves several chemical reactions and  different compounds known as clotting factors to form prothrombin activator. This series of chemical reactions in the coagulation phase is known as the coagulation cascade and defects at this point is one of the more common causes of blood clotting disorders.

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What is Prothrombin Time?

Prothrombin time (PT) is a means to measure the amount of prothrombin in the blood. Prothrombin is activated into thrombin when prothrombin activator acts on it. From here, thrombin then activates fibrinogen into fibrin. These steps are essential for blood clotting. Fibrin are the long protein strands that form a mesh network trapping blood cells and other blood components in order to form a blood clot. It serves to reinforce the existing platelet plug.

Simply, prothrombin time (PT) is a measure of the ability of the liquid consistency of the blood to begin clotting thereby forming into a semi-solid to solid consistency. When assessed with other similar tests, it is a good indication of the functioning of the blood’s clotting factors.

Partial Thromboplastin Time (PTT) vs Prothrombin Time (PT)

Another test known as the activated partial thromboplastin time (APTT) is also used to evaluate the blood coagulation ability. This test is also known as simply as the partial thromboplastin time (PTT) or sometimes as the partial thromboplastin time with kaolin (PTTK). While the prothrombin time (PT) tests the extrinsic pathway of the coagulation cascade, the partial thromboplastin time (PTT) tests the intrinsic pathway. The results of both tests together will allow for a collective assessment of the functioning of all clotting factors.

The prothrombin time (PT) will reveal deficiencies of factors II, V, VII, X or fibrinogen while the activated partial thromboplastin time (APTT) may indicate deficiencies of factors II, V, VIII, IX, X, XI, XII, fibrinogen deficiency, anti-factor antibodies like lupus anticoagulant.

The international normalised ratio (INR) is used to assess the therapeutic effect of coumarin anticoagulants. It is the ratio of the prothrombin time of the patient to a normal control. This is a standardized method to assess the effectiveness of blood thinning drugs like warfarin.

Prothrombin Time Test Procedure

Laboratory procedures may differ but the basic methodology involves the following :

  • Blood is removed from the patient.
  • A chemical with anti-clotting properties is added to prevent the withdrawn blood from clotting (oxalated).
  • Calcium and tissue factor is then added to the blood in order to test the clotting time. This activates the extrinsic pathway of coagulation.
  • The presence of calcium ions and tissue factor should activate the relevant clotting factors to convert prothrombin into thrombin.
  • The time taken for the blood to coagulate is recorded. This is the prothrombin time.

Foods and substances that may alter PT

  • Alcohol
  • Oral contraceptives
  • Hormone replacement therapy (HRT)
  • Vitamin K excess – nutritional supplements and foods like broccoli, chickpeas, kale, liver (offal) and soybeans.

Prothrombin Time Reference Values

Reference Range

The normal prothrombin time (PT) is approximately 9 to 12 seconds and the normal activated partial thromboplastin time (APTT) is approximately 26 to 36 seconds. The test result can either be normal or prolonged (sometime referred to as “high”).

Results

  • Normal PT :
    • With normal PTT, this means that the body’s ability to stop blood loss (hemostasis) is normal. It may be slightly prolonged yet there is no clotting factor deficiencies. Similarly the result may be normal while there may be slight deficiencies.
    • With prolonged PT, it may be an indication of deficiencies in clotting factors – VIII (hemophilia A), IX (hemophilia B), XI (hemophilia C), von Willebrand disease or lupus anticoagulant.
  • Prolonged PT :
    • With normal PTT, this means that there may be factor VII deficiency, liver disease or a vitamin K deficiency.
    • With prolonged PTT, there may be factor I, II, V or X deficiency, disseminated intravascular coagulation (DIC) or liver diseases (usually severe).

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