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Activated Partial Thromboplastin Time (aPTT)

A clotting-time test that shows how many seconds your blood takes to clot, and is also used to monitor heparin treatment.

AI-verified against cited sources. Awaiting expert reviewAlso called aPTT, APTT, PTTLOINC 14979-9

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What it is

The aPTT measures how long, in seconds, a sample of your blood takes to form a clot after the lab adds chemicals to start the process. It checks a group of clotting proteins, including factors VIII, IX, XI and XII. Each lab uses its own chemicals and machines, so the usual range differs from lab to lab. Some countries report an aPTT ratio (your time divided by a control time) instead of seconds.

Why it matters

A longer time can be a first hint that a clotting protein is low, that an antibody is getting in the way of the test, or that the blood thinner heparin is in the sample. Many of the common causes are not linked to bleeding. Hospitals also use the aPTT to adjust heparin treatment.

If it’s low

An aPTT shorter than the usual range is rarely a concern on its own. It is often related to how the blood was drawn or handled, or to a temporary rise in clotting factors.

Common causes

  • A hard or slow blood draw, or a sample that started to clot in the tube
  • Temporary rises in factor VIII during inflammation, stress, hard exercise or pregnancy
  • Normal differences between labs and reagents

What can help

  • Ask your clinician whether a repeat is useful if you were ill, stressed or pregnant when the blood was drawn

If it’s high

A longer aPTT means your blood took more time than usual to clot in the test tube. This does not always mean you are more likely to bleed. In one clinic study of people with an unexplained long aPTT, the most common causes were antibodies that disturb the test and low levels of certain clotting proteins, and the longest results were seen in people with those antibodies and in people without bleeding symptoms.

Common causes

  • Heparin (a blood thinner), including heparin picked up from an IV line or catheter during the blood draw
  • A sample problem, such as a tube that was not filled enough or a delay before testing
  • Antiphospholipid antibodies or lupus anticoagulant, which slow the test in the tube and are not usually a cause of bleeding (they can be linked to clotting problems instead)
  • Low factor XII or other 'contact' proteins, which often cause no bleeding
  • Low factor VIII, IX or XI, for example in hemophilia A or B, or in von Willebrand disease, which lowers factor VIII
  • Vitamin K deficiency or liver disease
  • Direct thrombin inhibitor medicines such as dabigatran
  • Widespread clotting and bleeding problems such as DIC (disseminated intravascular coagulation) in people who are seriously ill

What can help

  • Ask your clinician whether anything about the blood draw, your medicines or a recent illness could explain the result
  • Ask whether a repeat test or follow-up tests, such as a mixing study or clotting factor levels, make sense
  • Make sure everyone caring for you knows about all the medicines and supplements you take, including blood thinners

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 25–37 sec. Source: Mayo Clinic Laboratories (Extended Catalog), ARUP Laboratories, Biochemia Medica. Labs treat above 85 sec as urgent.

What can affect your result

  • The reagent and analyzer your lab uses, which set the usual range
  • Heparin and some other blood thinners, which lengthen the aPTT
  • How the sample was collected and handled: tube fill, hematocrit above 55%, and time before testing
  • Cloudy (lipemic) or hemolyzed samples
  • Inflammation, pregnancy and stress, which raise factor VIII and can shorten the aPTT
  • Heparin lengthens the aPTT. Blood drawn through an IV line or catheter can pick up heparin from the line and give a falsely long result.
  • Tubes that are not filled to the line (under about 90%), a very high hematocrit (above 55%), or long delays before the sample is spun can change the result. ARUP accepts room-temperature samples for 4 hours, and samples from people on heparin must be spun within 1 hour.
  • Lipemic (cloudy) or hemolyzed samples can give unreliable results on analyzers that read clotting by light.
  • Lupus anticoagulant antibodies and dabigatran lengthen the aPTT in the test tube.
  • High factor VIII, which rises with inflammation, stress, hard exercise and pregnancy, can shorten the aPTT.

Reflects clotting at the moment of the blood draw. Heparin changes it within hours.

When to talk to a clinician

Get urgent medical care for bleeding that will not stop, blood in your urine or stool, black stools, vomiting blood, or a severe headache after a head injury, whatever your aPTT is. ARUP's list of critical results includes a PTT of 85 seconds or more, so your lab or clinician may call you about a result that high. If you take heparin or another blood thinner, talk with your care team before changing anything.

Questions you could ask

  • Does my lab use the same reagent as for my earlier results, so the numbers can be compared?
  • Could a medicine, a blood thinner or the way the sample was collected explain this result?
  • Should we repeat the test, and would a follow-up such as a mixing study or clotting factor levels help?
  • Does this result change anything before a procedure or surgery?
  • Is a short aPTT worth following up, or could it be from how the sample was drawn?

Questions people ask

Why is my lab's usual range different from another lab's?

Labs use different chemicals and machines for this test, so the usual range in seconds is different for each one. Compare your result with the range printed on your own report.

My aPTT is long but I don't bleed easily. Should I worry?

A long result does not always point to a bleeding problem. Some common causes, such as lupus anticoagulant antibodies or low factor XII, lengthen the test without causing bleeding. It is still worth discussing with a clinician, who can look for the cause, especially before surgery.

Is the aPTT used to monitor every blood thinner?

No. It is used for heparin, although some labs, including Mayo, treat an anti-Xa test as the main method. Warfarin is followed with the INR. For newer blood-thinning pills the aPTT is not a reliable way to measure the effect.

Show the science and sources

Is a change real?

aPTT naturally varies about 2.8% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 9% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Systematic review and meta-analysis of within-subject and between-subject biological variation data of coagulation and fibrinolytic measurands (Hollestelle MJ et al.).

For clinicians and researchers

The aPTT is a clotting-time assay on citrated, platelet-poor plasma. A contact activator (silica, ellagic acid or kaolin) and phospholipid start the intrinsic pathway, and calcium then triggers clot formation. It is sensitive to deficiencies of factors XII, XI, IX and VIII (and prekallikrein and HMWK), less so to common-pathway factors, and to inhibitors such as heparin, lupus anticoagulant, direct thrombin inhibitors and specific factor inhibitors. Reagents differ widely in their sensitivity to factor deficiencies, heparin and lupus anticoagulant, so intervals in seconds are not interchangeable between labs. When the interval is expressed as an aPTT ratio it is generally about 0.8–1.2. For unfractionated heparin, each lab should set its therapeutic aPTT range against anti-Xa activity of about 0.3–0.7 IU/mL, and Mayo names heparin anti-Xa as the primary method for monitoring. Dabigatran lengthens the aPTT with a non-linear dose response, so the aPTT cannot quantify it. In a series of 187 outpatients with an unexplained long aPTT, 22.6% had antiphospholipid antibodies, 17.4% contact-pathway factor deficiency, 11.6% other factor deficiencies, 11.6% vitamin K deficiency or liver disease, and 22.1% had no cause found. Pre-analytical points include 3.2% citrate, tube fill within 90–110% of the line, citrate adjustment above 55% hematocrit, double centrifugation before freezing, heparin contamination from catheters, and hemolysis or lipemia on optical analyzers.

LOINC 14979-9 (primary), 3173-2. Specimen: plasma.

Sources

  1. Activated Partial Thromboplastin Time, Plasma (test APTTP), Mayo Clinic Laboratories (Extended Catalog)
  2. Partial Thromboplastin Time (ARUP test 0030235), ARUP Laboratories
  3. Critical Values List (CORP-APPEND-0104A, Rev. 46, April 2026), ARUP Laboratories (2026)
  4. Croatian Society of Medical Biochemistry and Laboratory Medicine: National recommendations for blood collection, processing, performance and reporting of results for coagulation screening assays prothrombin time, activated partial thromboplastin time, thrombin time, fibrinogen and D-dimer, Biochemia Medica (2019)
  5. Prolonged APTT of unknown etiology: A systematic evaluation of causes and laboratory resource use in an outpatient hemostasis academic unit, Research and Practice in Thrombosis and Haemostasis (2019)
  6. Measurement of non-VKA oral anticoagulants versus classic ones: the appropriate use of hemostasis assays, Thrombosis Journal (2014)
  7. WFH Guidelines for the Management of Hemophilia, 3rd edition: Chapter 3, Laboratory Diagnosis and Monitoring, World Federation of Hemophilia (Haemophilia) (2020)
  8. PTT (Partial Thromboplastin Time) Test, MedlinePlus (National Library of Medicine)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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