Prothrombin time (PT) and partial thromboplastin time (PTT) are tests used to evaluatecoagulation, or the clotting of blood. While both measure how quickly blood clots, they have distinct purposes:
When looked at individually and together, the PT and PTT can reveal ableeding disorderand your risk for severeblood clots. It can also show how well your blood clots in advance of surgery and how well you are responding toanticoagulant therapy(“blood thinners”).
This article explains how the PT and PTT tests work, what they indicate, and how the test results are interpreted.
Verywell / JR Bee
What Are Platelet Disorders?
Prothrombin Time (PT)
Prothrombin, also known as clotting factor 2, is one of 13 substances known as “clotting factors” that are involved in coagulation. How fast blood clots after bleeding depends on how much of each clotting factor is in the blood.
Based on the clotting time of the PT test, a lab may be able to determine:
How the Test Works
Once blood is drawn, a substance called a tissue factor is added to the test tube. Tissue factor, also known as clotting factor 3, activates the sample in a way that illustrates how blood would clot if there is bleeding (the extrinsic pathway).
Once activated, the PT test is measured in seconds. The time is then compared to the reference range of values (meaning the time span in which blood clotting is considered normal).
With the PT test, the reference range is between 11 to 13.5 seconds if you arenoton anticoagulants.
A number higher than the reference range means your blood is taking longer than usual to clot. A number lower than the reference range means that your blood is clotting faster than normal. There are many possible reasons why these might occur.
One of the key reasons is that you may have too much or too littlevitamin Kin your blood, which your body uses to build clotting factors.
Other medical conditions, substances, and diseases can also affect normal blood clotting.
Clotting Too QuicklyHigh Vitamin K intakeEstrogen-containing drugsDeep vein thrombosis (DVT)Pulmonary embolism (PE)Clotting Too SlowlyVitamin K deficiencyAnticoagulant therapyLiver diseaseHemophiliaVon Wildebrand diseaseSickle cell disease
Clotting Too QuicklyHigh Vitamin K intakeEstrogen-containing drugsDeep vein thrombosis (DVT)Pulmonary embolism (PE)
High Vitamin K intake
Estrogen-containing drugs
Deep vein thrombosis (DVT)
Pulmonary embolism (PE)
Clotting Too SlowlyVitamin K deficiencyAnticoagulant therapyLiver diseaseHemophiliaVon Wildebrand diseaseSickle cell disease
Vitamin K deficiency
Anticoagulant therapy
Liver disease
Hemophilia
Von Wildebrand disease
Sickle cell disease
The PT test does not require fasting but foods high in vitamin K can skew the results, including beef or pork liver, dark green vegetables, or soybeans.
International Normalized Ratio (INR)
The PT test is sometimes referred to as the PT/INR test. The INR refers to the international normalized ratio, a calculation that helps ensure test results are standardized from one lab to the next.
Because there are variations in the different types of tissue factor used for the PT test, the INR calculation ensures that the clotting time is accurate based on the body of test results from tissue factor manufacturers worldwide.
INR values are important because they help determine how well a person is responding to warfarin, one of the most commonly prescribed anticoagulants used to prevent blood clots.
For people on warfarin, the reference range of values for PT/INR is 2 to 3 seconds.
High and low INR values are interpreted as follows:
Based on the values, a healthcare provider can decide if the warfarin dose needs to be adjusted or if other interventions are needed.
Partial Thromboplastin Time (PTT)
The partial thromboplastin time (PTT) test also measures the speed of clotting but differs from the PT test in that it aims to establish how blood clots within a blood vessel (intrinsic pathway). This is based in part on an enzyme calledthromboplastin(clotting factor 11) that converts prothrombin into its more active form, called thrombin.
In looking at how clotting works within a blood vessel, the PTT test can help determine:
How It Works
The processes involved in the PTT test are similar to the PT test, but, instead of using tissue factor, the blood sample is activated with minerals or acids that show how blood clotting occurs when something is introduced into the blood.
With the PTT test, the reference range is between 25 and 33 seconds.
As with the PT test, a higher PTT number means your blood is taking longer than usual to clot. A lower PTT number means that your blood is clotting faster than normal. There are several reasons why this might occur.
Clotting Too QuicklyAn advanced whole-body infection that is using up clotting factor too quicklyMetastaticovary, colon, or pancreatic cancerClotting Too SlowlyVitamin K deficiencyToo much heparinLiver diseaseHemophiliaVon Wildebrand diseaseLupusAntiphospholipid syndrome (APS)Certain types ofleukemia
Clotting Too QuicklyAn advanced whole-body infection that is using up clotting factor too quicklyMetastaticovary, colon, or pancreatic cancer
An advanced whole-body infection that is using up clotting factor too quickly
Metastaticovary, colon, or pancreatic cancer
Clotting Too SlowlyVitamin K deficiencyToo much heparinLiver diseaseHemophiliaVon Wildebrand diseaseLupusAntiphospholipid syndrome (APS)Certain types ofleukemia
Too much heparin
Lupus
Antiphospholipid syndrome (APS)
Certain types ofleukemia
Heparin also differs in that interferes with the action of thromboplastin, whereas warfarin interferes with the action of vitamin K.
By understanding the nature of intrinsic blood clotting, a surgeon can determine how much heparin is needed for surgery for each individual patient.
Risks and Complications
Blood draws are routine, low-risk procedures. While rare, it is possible to have complications from a blood draw, including:
If you feel lightheaded during or after a blood draw, let one of the medical staff know.
Keep the injection site clean, and call your doctor if you have signs of infection (including fever, chills, or increasing pain, redness, warmth, or swelling at the injection site).
Summary
The prothrombin time (PT) test and partial thromboplastin time (PTT) measure how quickly blood clots. The tests can help detect bleeding disorders, check a person’s response to blood thinners, and assess a person’s risk for bleeding prior to surgery.
The PT test is useful in evaluating the effects of the anticoagulant warfarin and identifying clotting and bleeding disorders. The international normalized ratio (INR) test ensures that PT results are standardized.
The PTT is mainly used to monitor a person’s response to the anticoagulant heparin and can also identify diseases that interfere with blood clotting.
4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
UCSF Health.Prothrombin Time (PT).
MedlinePlus.Prothrombin time test and INR (PT/INR).
UCSF Health.Partial thromboplastin time (PTT).
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