Why Is My PTT Prolonged? Clotting Tests Explained

A prolonged PTT means your blood took longer than the lab's reference range to form a clot in a test tube. Common explanations include heparin treatment, a clotting factor deficiency, liver disease, vitamin K deficiency, or a problem with the sample itself. Some causes raise bleeding risk, while others, such as lupus anticoagulant, may be linked to clotting. This article explains what the test measures, the usual causes, how clinicians work it up, and which symptoms need urgent care.

What a prolonged PTT result actually measures

PTT stands for partial thromboplastin time. Many labs report it as aPTT, the activated version. The test adds reagents to a sample of your plasma and times how long a clot takes to form. It looks at the "intrinsic" and "common" parts of the clotting cascade, which include factors VIII, IX, XI and XII, along with factors X, V, II and fibrinogen.

A result above the lab's reference range is called prolonged. Reference ranges differ between labs and between the reagents they use, so the number printed next to your result is the one that matters. A prolonged PTT is a laboratory finding. It is not a diagnosis, and by itself it does not tell you whether you are at higher risk of bleeding.

Clinicians usually read the PTT together with the PT/INR, a platelet count, and your history. If you want background on a related result, see the article on an elevated INR with warfarin. This page focuses on the PTT.

Why is my PTT prolonged? The common explanations

Several factors can contribute, and more than one can be present at once. A published review of isolated aPTT prolongation, listed in Sources, emphasizes that a prolonged result does not always mean a bleeding problem.

Heparin and other medicines

Unfractionated heparin is a common reason for a prolonged PTT, and hospitals often use the PTT to monitor it. Some other anticoagulants, including direct thrombin inhibitors, can also lengthen the PTT. If you take a blood thinner, tell the clinician reviewing your result, and mention every medicine and supplement you use, including aspirin and other anti-inflammatory pain relievers (NSAIDs such as ibuprofen), which can add to bleeding risk. Do not start, stop or change the dose or timing of any anticoagulant or other prescription on your own. Ask your prescriber or pharmacist, and read the product labeling for boxed warnings about bleeding.

A problem with the sample

Blood drawn through a line that contained heparin, a tube that was underfilled, or a sample with a high red blood cell concentration can produce a falsely long result. Because of this, clinicians often repeat an unexpected isolated result before acting on it.

Clotting factor deficiencies

Inherited deficiencies of factor VIII (hemophilia A), factor IX (hemophilia B) or factor XI can prolong the PTT. MedlinePlus describes hemophilia as a disorder in which blood does not clot normally. Von Willebrand disease can sometimes prolong the PTT too, although a normal PTT does not rule it out.

Liver disease and vitamin K deficiency

The liver makes most clotting factors. When liver function is reduced, both the PT and PTT may lengthen. Vitamin K is needed for several factors, and low levels can occur with poor absorption, long antibiotic courses, or limited intake. Research on vitamin K deficiency in pregnancy and on newborn hemostasis, listed in Sources, shows why age and life stage matter when interpreting these tests.

Severe illness and injury

Sepsis and major trauma can disturb clotting, and research on injured patients shows that PT and PTT changes are not fully explained by factor levels alone. The mechanism is not fully understood and likely involves several processes.

Lupus anticoagulant

Lupus anticoagulant is an antibody that interferes with the test reagents. Despite the name, it is not limited to people with lupus. It can prolong the PTT in a tube while being associated with a higher risk of clots in some people, not bleeding. This is why a prolonged PTT does not always signal a bleeding tendency, and why a clinician needs to sort out the cause before anyone changes a blood thinner or other treatment.

Factor inhibitors

Rarely, the immune system makes antibodies against a clotting factor, most often factor VIII. This is a much less common cause than the everyday ones above, and a single prolonged result does not establish it. It is typically considered when the PTT stays prolonged and bleeding symptoms appear without a family history.

How clinicians work it up

Clinicians combine your history, a physical examination and testing. Questions often cover bleeding with surgery or dental work, heavy periods, easy bruising, nosebleeds, family history, liver disease, and every medicine and supplement you take, including blood thinners.

A common next step is a mixing study. Your plasma is mixed with normal plasma and the PTT is repeated. If the time corrects, a factor deficiency is a possible explanation. If it does not correct, an inhibitor such as lupus anticoagulant or a factor antibody may be considered. Further tests can include individual factor levels, a PT/INR, fibrinogen, liver tests and lupus anticoagulant testing. A platelet count, which is part of a complete blood count, is also usually checked. See the article on low platelet counts for that topic.

Does a prolonged PTT mean I will bleed?

Not necessarily. Bleeding risk depends on the cause and on your symptoms. A person with lupus anticoagulant may have a long PTT and no bleeding tendency. A person with a mild factor deficiency may bleed after surgery despite a modest result. This is why clinicians weigh the number against how you feel and what has happened in the past.

MedlinePlus describes bleeding disorders as conditions in which blood does not clot properly, and lists symptoms such as unusual bruising, bleeding that is hard to stop, and heavy periods. If you notice these, mention them along with your result.

Bleeding is not always visible. In people with a bleeding disorder or on a blood thinner, bleeding can occur deep in the body, for example in the abdomen or back, inside a muscle of an arm or leg, or in the neck where swelling can press on the airway. Sudden severe abdominal, back or flank pain, a rapidly swelling, tense and painful limb, or swelling of the neck or throat needs emergency care, as listed in the box below.

Prolonged PTT before surgery or a procedure

A prolonged PTT is sometimes found on routine preoperative bloodwork in a person with no symptoms. Often the surgical team will repeat the test and look for a cause before proceeding. Tell them about any history of bleeding after dental work or injuries, and about family members with bleeding problems. They may involve a hematologist, a doctor who specializes in blood disorders.

PTT versus D-dimer and clotting

A prolonged PTT and an elevated D-dimer answer different questions. The PTT measures how long clot formation takes in the lab. D-dimer reflects clot breakdown. The related article on D-dimer explains that result. If you have leg swelling or pain, see the MedlinePlus page on deep vein thrombosis for symptoms to know.

What you can do while you wait for answers

• Bring a list of every medicine, vitamin and supplement, including all blood thinners, aspirin and other over-the-counter pain relievers, so your clinician can review them.
• Write down bleeding history, including nosebleeds, bruising, periods, and bleeding after procedures.
• Ask your clinician whether the test should be repeated and what follow-up tests are appropriate for you.
• Tell your clinician before any planned surgery, dental work or injection that you have a prolonged result.
• Do not start or stop any anticoagulant, aspirin or NSAID without medical advice, and keep taking prescribed medicines unless your prescriber tells you otherwise.

In many cases, the cause can be identified with a few additional tests, and treatment depends on the cause.

When to Seek Medical Care