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Partial Thromboplastin Time (PTT) Blood Test: What Does It Tell You?

Medically Approved by Dr. Edward Salko

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When it comes to detecting blood clotting abnormalities, you will likely need to undergo a PTT blood test to evaluate how long your blood clots. Test values beyond the normal PTT levels may suggest blood disorders, autoimmune diseases, liver disease, and excessive heparin. Learn more about this test and how prolonged clotting affects your health severely. 

What Is the PTT Blood Test Used For? 

A partial thromboplastin time (PTT) blood test measures the time it takes for blood to clot. It’s a standard lab procedure for detecting bleeding disorders and monitoring the effect of blood thinners. 

Clotting or coagulation occurs when the blood changes from a liquid to a gel-like consistency. This prevents further bleeding. Otherwise, your body could lose substantial blood to the point where it becomes life-threatening. 

Once a blood vessel is damaged or punctured, the blood coagulates and forms clots, primarily made of a special protein called fibrin. The clot blocks the hole, preserving blood plasma and promoting tissue repair. 

While blood clot formations are natural and designed to prevent profuse blood loss (hemorrhage), in the absence of bleeding, it can become a severe medical case by itself. Blood clots can form blockages in the blood vessels, blocking blood flow – a condition called thrombosis.

When To Take It

Your healthcare provider will likely order a PTT blood test following these conditions and symptoms:

  • Presence of bleeding or bruising
  • Post-surgical check
  • Blood thinner prescription
  • Swelling in the legs
  • Shortness of breath
  • Frequent nose bleeds
  • Heavy menstrual bleeding

How To Prepare for the Test

Before taking a PTT blood test, you don’t have to undergo special preparations, such as fasting. If you’re getting tested for suspected bleeding and have been taking blood thinners, you should inform your doctor and the lab technician. 

Also, a PTT blood test requires a specific amount of blood sample, which is typically drawn via venipuncture. Therefore, it has to be taken in a medical laboratory.

Pro Tip: If you’re anxious about undergoing PTT because of the venipuncture procedure, check out our tips on how to prepare when having your blood drawn


An activated partial thromboplastin time (aPTT) test differs from a PTT test only in that an activator is added to make the clotting time faster. With the PTT test, the reference interval is between 60 to 70 seconds, while aPTT only has 30 to 40 seconds. 

Both tests follow the same procedure in detecting bleeding disorders and monitoring anticlotting or anticoagulant drug effects. However, aPTT is more sensitive than PTT, making it more ideal in specific cases, such as heparin regulation. Nonetheless, PTT and aPTT are commonly interchangeable in evaluating blood disorders. 

PTT Blood Test Results Meaning 

For the blood test to show normal results, your PTT lab values should not exceed 70 seconds; otherwise, it’s classified as prolonged PTT. On the other hand, the aPTT blood test’s normal range is between 30 to 40 seconds. Going beyond the ceiling of the normal aPTT or PTT level’s normal range indicates hematological issues, nutrient malabsorption, and other physiological problems.

On the other hand, if your test results fall on PTT’s normal values, it confirms normal coagulation. However, in some cases, your doctor may request other tests for extra measures, most notably the prothrombin test (PT), also known as the international normalized ratio (INR). This test also measures blood clot time. 

What Does It Mean When the PTT Is Prolonged? 

A PTT that takes longer than normal may suggest the following conditions: 

Blood disorders

Several abnormalities in the blood can lead to prolonged or high PTT in a blood test, including leukemia. Others that are more pronounced involve a coagulation factor deficiency, such as these disorders:

  • von Willebrand disease: A condition that directly inhibits blood clotting due to low or malfunctioning Willebrand factor.
  • Hemophilia A and B: Hemophilia is an inherited blood disorder that usually affects a male’s blood clotting ability, although females can be carriers. It can be classified into two based on the missing factor. For example, coagulation factor VIII is lacking in hemophilia A, while hemophilia B has factor IX deficiency. 
  • Disseminated intravascular coagulation (DIC): DIC is a rare blood clotting disorder resulting from an infection or injury severe enough to cause significant damage or extreme immune response. Aside from elevated PTT, a low platelet checked via complete blood count can also be observed. 

Autoimmune diseases

Certain autoimmune disease leads to prolonged PTT. Most notably, this includes the antiphospholipid antibody syndrome, wherein the body produces excessive antibodies (proteins that help eliminate foreign substances). Some of these antibodies induce increased clotting, but PTT is slower than normal. A PTT blood test can be part of the autoimmune disease tests

Liver disease

Liver cells help produce coagulation factors and other proteins involved in blood clot formation. Therefore, abnormalities in coagulation can be associated with liver disease. Additionally, as a result, liver profile blood tests are also sometimes accompanied by a PTT blood test.

Did You Know? 90% of liver disease is due to lifestyle factors. Know the 10 habits that destroy your liver and find out how to avoid them. 

Frequently Asked Questions

What is the difference between PT and aPTT?

Prothrombin time (PT) is different from activated partial thromboplastin time (aPTT) in that PT measures coagulation based on the extrinsic pathway, while aPTT measures blood clotting based on the intrinsic pathway. These pathways are determined by factors that activate the conversion of fibrinogen to fibrin. In an extrinsic pathway, coagulation is triggered by blood factors, while the intrinsic pathway is activated through tissue factors. 

A PT test is commonly performed to check on the impact of blood-thinning medications. On the other hand, an aPTT or PTT blood test will be ordered to evaluate blood disorders. 

Why are PT and PTT important?

PT or INR and PTT are the hallmarks of detecting blood clot disorders and monitoring the effectiveness of anticoagulant drugs and therapies. Blood tests measuring both parameters provide important data on how long your blood clots are and its implication for your health. In addition, it helps detect bleeding, which then allows for speedy medical intervention.  

Is PTT a liver function test?

A PTT or aPTT blood test can provide information about liver function. Slower coagulation can be associated with a shortage in blood coagulation factors, such as fibrinogen and prothrombin – proteins synthesized by the liver cells. Problems involving liver function affect the production of these proteins. Thus, healthcare providers consider a PTT blood test a good reference for liver function assessment.  

Does dehydration affect aPTT?

Dehydration increases your risk for blood clots, especially when accompanied by other medical conditions like renal failure. Its impact can be detected through the aPTT blood test. Hence, medical experts recommend regular testing for monitoring anticoagulant treatment, including warfarin intake, if dehydration is deemed critical by your doctor. 

Does vitamin K improve aPTT?

Vitamin K is vital in synthesizing coagulation factors produced in the liver. Hence, its sufficiency impacts aPTT. Those with vitamin K deficiency are susceptible to prolonged blood clotting. Thus, healthcare providers check and recommend foods high in vitamin K for those with blood clot-related disorders, including leafy greens, dairy products, and fortified foods. 

The Bottom Line

Getting a PTT or an aPTT blood test will allow your doctor to diagnose certain blood disorders and check on how anticoagulants work. However, keep in mind that blood clotting problems can be connected to an underlying condition or can be confounded by other symptoms. That’s why it’s best to follow through with your doctor’s recommendation for additional testing, like a D-dimer test, treatment, or lifestyle adjustments. 

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