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Normal prothrombin time in a blood test

Blood test
Normal prothrombin time in a blood test
Last update: 19/05/2021

What is the normal value of prothrombin time in a blood test?

Prothrombin time in a blood test is age-dependent:

Adults: 11 - 13 seconds
Children from 11 to 16 years old: 12.7 - 16.1 seconds
Children from 6 to 10 years old: 11.5 - 15.1 seconds
Children from 1 to 5 years old: 12.1 - 14.5 seconds
Babies up to 1 year old: 11.5 - 15.3 seconds
Newborns: 13.5 - 16.4 seconds

Why normal levels can differ across different labs?

Each laboratory must establish its own normal range for prothrombin time in a blood test. These ranges depend on the makeup of the local population, the technologies used and the accuracy of the measurement. There may be also slight differences in the normal levels according to age, gender, race or ethnic origin, geographic region, diet, type of sample and other relevant status.

Your doctor will study the results along with your medical record, screenings, physical condition, symptoms and any other relevant information about your situation.

Thera are some circumstances that may alter the normal ranges described above:

  • There are different reagents (tissue thromboplastin) that may be added to the blood sample to perform the prothrombin time test. For this reason, the normal range may be differ from lab to lab depending on the reagent used.
  • During pregnancy, the prothrombin time may be a bit lower. It may considered within the normal range values between 9.5 and 13.5 seconds.

What does the prothrombin time mean?

Prothrombin time (PT) is a measurement of the amount of time needed for a clot to form after the addition of a tissue factor (tissue thromboplastin) and calcium to a sample of platelet-poor plasma.

Therefore, the prothrombin time (PT) is a test that measures the efficacy of the extrinsic coagulation process (clot formation and stop bleeding). Formation of a clot involves the sequential interaction of series of plasma proteins, called coagulation factors, in a highly ordered and complex manner and also the interaction of these factors with blood platelets and materials released from the tissues.

Prothrombin time is a direct measure of the conversion of prothrombin into thrombin and correct activity of plasma coagulation factors I, II, V, VII and X.

Coagulation factors are proteins produced by the liver (except VIII factor) and depend on adequate vitamin K intake and absorption.

What is the prothrombin time blood test used for?

The prothrombin time (PT) test is one of the most important screening tests used in diagnostic coagulation studies. Prothrombin times may be used to screen for initial coagulation disorders. It is also interesting to know the activated partial thromboplastin time (aPTT) test for a better diagnosis.

The prothrombin time (PT) test is often performed as part of a coagulation screen before surgical procedures or when a patient shows signs of excessive bleeding or bruising (nosebleeds, bleeding gums, or blood in urine or stool).

The prothrombin time (PT) is also used to help monitor oral anticoagulant use, as the result is elevated when a patient is being treated with these oral anticoagulants. These drugs (warfarin, for example) are used by patients who are at a high risk for blood clot formation.

As coagulation factors are produced by the liver, patients suffering from liver disease (hepatitis, cirrhosis, etc.) may also have an increased prothrombin time result because of the inability of the liver to create the clotting factors as needed.

A prothrombin time (PT) higher than normal may be a sign of:

A prothrombin time (PT) lower than normal has no clinical meaning and it is not used for diagnosis. Anyway, it may a sign of:

  • Thrombophlebitis.
  • Malignant tumors.

Where can I find more information about prothrombin time in a blood test?

You can visit our pages about:

Which values are considered a normal prothrombin time in a blood test?

The following values are considered to be normal values:

IMPORTANT: These levels are expressed in seconds. They are an example of a healthy man/woman of about 40 years old with no known disease and not taking any medication. The ranges can be different depending on the laboratory or on your personal circumstances.

Prothrombin time
11 s11.1 s11.2 s11.3 s11.4 s11.5 s11.6 s11.7 s
11.8 s11.9 s12 s12.1 s12.2 s12.3 s12.4 s12.5 s
12.6 s12.7 s12.8 s12.9 s13 s   
Medically reviewed by our Medical staff on 19/05/2021


  • Concise Book of Medical Laboratory Technology: Methods and Interpretations. 2nd Edition. 2015. Ramnik Sood. ISBN: 978-93-5152-333-8. Pag. 283.
  • A Manual of Laboratory and Diagnostic Test. 9th edition. Frances Fischbach. Marshall B. Dunning III. 2014. Pag 158. ISBN-10: 1451190891.
  • Laboratory tests and diagnostic procedures with nursing diagnoses (8th ed), Jane Vincent Corbett, Angela Denise Banks, ISBN: 978-0-13-237332-6, Pag. 300.
  • Essentials of Medical Laboratory Practice. Constance L. Lieseke, Elizabeth A. Zeibig. 2012. ISBN: 978-0-8036-1899-2 Pag: 319.

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