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What is PT test?

PT refers to prothrombin time test or pro time test. Prothrombin is a clotting factor synthesised by the liver with the help of vitamin K. Clotting is the process of congealing of blood at the site of injury by protein fibres and chemical factors present in it.

A cascade of reactions begins in blood when an injury damages a blood vessel. Blood clots at the location of the injury and a solid blockade forms at the point of damage, ceasing bleeding within a few seconds.

Time is an essential factor in the clotting process to prevent excess loss of blood from the body. Inadequate functioning of any of the clotting factors can interfere with the clotting process and put an individual at risk of excessive blood loss.

Prothrombin time test measures the time taken by a blood sample to clot. Typically, a blood sample should clot within seconds. This time could be higher if there are any pre-existing conditions the individual is diagnosed with. If an individual is on medications that affect the process of clotting, e.g., warfarin, it can also increase clotting time.

  1. Why is PT test performed?
  2. How do you prepare for PT test?
  3. How is PT test performed?
  4. What do PT test results mean?

A medical practitioner may order for a PT test in case of:

A common practice is to regularly check the effect of any blood-thinning/anticoagulant medications a person is taking with the help of a  PT test. E.g., warfarin is a blood-thinning medication and an anticoagulant given to individuals with disorders like thrombophilia, pulmonary embolism or abnormal heartbeat. It slows down clotting process and keeps the blood in a liquid form for a longer time. Thus, taking warfarin would alter the clotting time to a specific recommended range.

Not much preparation is required for PT test. Inform the doctor about any medications or herbal supplements you may be taking as they may be required to be stopped before the test. Do not change or stop the course of medication without consulting the doctor.

For a PT test, blood is withdrawn from a vein in the arm or hand using a sterile needle. The prick of the needle may cause discomfort for a few seconds, which should disappear subsequently. Once a blood sample is withdrawn, it is deposited in a sterile vial. The puncture on the skin is covered with cotton gauze and bandage. There may be a bit of bruising on the skin at the point of needle insertion, which will resolve soon.

Normal results:

PT test result is generally given in terms of international normalised ratio (INR) for standardisation across laboratories. The test results can also be presented in seconds.

Normal values for any individual can vary in the INR range 0.8-1.10. These values correspond to approximately 11-13.5 seconds (time taken by the blood to clot).

Results in these ranges mean that blood is clotting within a standard time range and the concentration of functional prothrombin in blood is appropriate. PT test is not an absolute test for determining the risk of excess bleeding. Other factors may also be responsible for reduced blood clotting.

Abnormal results:

When results are above the range of normal, it means that blood is taking longer time to clot and the individual may be at risk of excessive bleeding. The cause of slower blood clotting could be:

  • Use of blood-thinning medications such as warfarin in individuals who  have hypercoagulable disorders (conditions that cause increased clotting of the blood)
  • Liver dysfunction such as cirrhosis
  • Vitamin K deficiency
  • Bleeding disorders like haemophilia
  • Disseminated intravascular coagulation

Other tests may be ordered by the medical practitioner to confirm the causes mentioned above.

It is essential for individuals who are on blood-thinning medications to know if their results are outside the range of INR 2.0-3.0 as they may require a dose alteration. Alteration of dose should be done only after consulting the doctor. Other causes of such a result could be:

  • Vitamin supplements that interfere with the functioning of blood-thinning medications
  • Alcohol intake
  • Certain leafy vegetables like spinach and kale

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. This information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.

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  1. Lab Tests Online-Au; Australasian Association of Clinical Biochemist; Hypercoagulable disorders
  2. Guyton A.C, Hall J.E. Textbook of medical physiology. 11th edition. Pennsylvania: Elsevier Saunders; 2016. Chapter 36, Hemostasis and Blood Coagulation; p. 457-468.
  3. National Health Service [internet]. UK; Warfarin
  4. Ortel TL. Antithrombotic therapy. McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St Louis, MO: Elsevier; 2017:chap 42.
  5. MedlinePlus Medical Encyclopedia: US National Library of Medicine; Prothrombin time (PT)