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Blood Test Tracker

Prothrombin Time (PT)

How many seconds your blood plasma takes to clot in the lab. It checks several clotting proteins and is used to follow warfarin.

AI-verified against cited sources. Awaiting expert reviewAlso called PT, Pro TimeLOINC 5902-2

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What it is

Prothrombin time (PT) measures how long a sample of your blood plasma takes to form a clot after the lab adds tissue factor and calcium. It checks clotting proteins made in the liver, including factors II, V, VII and X and fibrinogen. Several of them need vitamin K to work. The result is in seconds. Labs use different reagents and analyzers, so the usual range changes from lab to lab. Many reports also show an INR, a number that adjusts the PT so results can be compared between labs for people who take warfarin.

Why it matters

The PT helps show whether your blood clots at a usual speed. A longer PT can be a sign of low vitamin K, liver disease, a shortage of a clotting protein, or a blood thinner such as warfarin. Clinicians use it to look into unexplained bleeding or bruising, to check clotting before some procedures, and alongside liver tests. For people on warfarin, the PT is turned into an INR to guide care. The number alone does not explain the cause, so it is read together with the aPTT, your history and your medicines.

If it’s low

A PT shorter than your lab's range is uncommon and is usually not a concern on its own. It can happen with vitamin K supplements or a diet high in vitamin K, and with certain medicines. Rarely it goes along with a condition that makes blood clot too easily, so your clinician will look at your other results.

Common causes

  • Vitamin K supplements or a diet high in vitamin K
  • Certain medicines
  • Natural variation near the edge of the range
  • A clotting disorder in which blood clots too easily (uncommon)

What can help

  • Tell your clinician about every supplement and medicine you take, including vitamin K and multivitamins
  • Ask whether any follow-up is needed

If it’s high

A longer PT means your blood took more time to clot than your lab's usual range. Warfarin does this on purpose. Other common reasons are low vitamin K and liver problems, and some other blood thinners can lengthen it too. A tube that was underfilled can also make the result look longer. Your clinician can work out which one applies to you.

Common causes

  • Warfarin or another vitamin K blocker (the expected effect)
  • Low vitamin K, from poor intake or poor absorption of fat
  • Liver disease
  • Other blood thinners, including some direct oral anticoagulants and high-dose heparin
  • A sample problem, such as an underfilled tube or a delay before testing
  • A shortage of a clotting protein, such as factor VII, which is uncommon

What can help

  • Ask your clinician whether a repeat test, an INR or an aPTT would help
  • If you take warfarin, follow your clinic's plan for testing, and check with them before making any changes
  • Ask how to keep your vitamin K intake steady, because big swings in leafy greens or supplements can change results

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: 9.4–12.5 seconds. Source: Mayo Clinic Laboratories (Extended Catalog, via testcatalog.org), ARUP Laboratories, Indiana University Health Pathology Laboratory, Practical-Haemostasis.com. Labs treat above 46.4 seconds as urgent.

What can affect your result

  • Warfarin and other blood thinners
  • Vitamin K in food and supplements, and alcohol (these matter most if you take warfarin)
  • Liver health
  • The reagent and analyzer your lab uses, which sets the usual range
  • How full the blood tube is, a very high hematocrit, and how quickly the sample is processed
  • Age: newborns have lower vitamin K–dependent clotting proteins and use their own ranges
  • Warfarin and other vitamin K blockers lengthen the PT. This is the intended effect, and the INR is used to monitor it.
  • Some other blood thinners can lengthen the PT depending on the reagent, including high levels of unfractionated heparin, direct thrombin inhibitors such as dabigatran, and factor Xa inhibitors.
  • Vitamin K in food and supplements, alcohol, high-fat diets and soybean or canola oil can change results, which matters most if you take warfarin.
  • An underfilled blood tube (too much citrate for the amount of blood) or a very high hematocrit (above 0.55) lengthens clotting times unless the tube is adjusted. Delays of more than about 4 hours before the sample is processed can also affect results.
  • Platelets left in the plasma can give spurious results, which is why labs spin the sample twice.

If you take warfarin, follow your clinic's instructions about when to have the test. The PT reflects your clotting proteins at the time of the draw. Warfarin changes it over several days.

When to talk to a clinician

Call the clinician who ordered the test the same day if your lab flagged the PT or INR as urgent, or if you have unexplained heavy bleeding or bruising. One large reference lab (ARUP) treats a PT of 46.4 seconds or higher as an urgent result, but limits differ by lab and reagent. Call 911 or your local emergency number for bleeding that will not stop with firm pressure, a head injury while on a blood thinner, vomiting blood, black or bloody stools, or a sudden severe headache.

Questions you could ask

  • Which range does my lab use for PT?
  • If I take warfarin, how often should my INR be checked?
  • Could a medicine, supplement or my diet explain a longer PT?
  • Should we look at my liver tests, my aPTT or my vitamin K status?
  • Should this test be repeated?
  • Could a supplement or medicine I take explain a shorter PT?

Questions people ask

Why does my PT range look different from a friend's?

Each lab uses its own reagent and analyzer. Published adult ranges run from about 9 to about 15 seconds. Compare your result with the range printed on your own report.

What is the difference between PT and INR?

PT is the clotting time in seconds. INR is a calculated number that adjusts the PT for the reagent so results from different labs can be compared for people who take warfarin. Your clinician decides which number matters for you.

What do "Test" and "Control" mean on my report?

Some labs, including many in India, print the PT as "Test" and "Control". Your result is the Test value. The Control shows what a normal sample did on the same day. A Test value well above the Control means a longer clotting time.

Show the science and sources

Is a change real?

PT naturally varies about 2.6% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 8% 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

PT is a one-stage clotting time on citrated, platelet-poor plasma. It is started by adding tissue factor (recombinant or tissue-derived, depending on the reagent), phospholipid and calcium, and it is sensitive to factors VII, X, V and II and fibrinogen. Every reagent and analyzer pair has its own International Sensitivity Index (ISI) and mean normal PT, so reference intervals vary widely (for example 9.4–12.5 s at Mayo, 9.3–13.5 s at IU Health and 12.0–15.5 s at ARUP). INR = (patient PT ÷ mean normal PT)^ISI. It was calibrated in people on vitamin K antagonists, so it is meant for warfarin monitoring. An isolated long PT with a normal aPTT points toward factor VII deficiency, early warfarin effect or early vitamin K deficiency. A long PT with a long aPTT suggests a shortage across several factors or an inhibitor, and a 1:1 mixing study helps separate the two. Heparin, direct thrombin inhibitors and factor Xa inhibitors affect the PT in a reagent-dependent way. ARUP's critical-value list sets an urgent-call limit at 46.4 s or higher.

LOINC 5902-2 (primary), 42638-7, 34528-0. Specimen: plasma.

Sources

  1. Prothrombin Time, Plasma (test ID PTTP), Mayo Clinic Laboratories (Extended Catalog, via testcatalog.org)
  2. Prothrombin Time (test 0030215), ARUP Laboratories
  3. Thrombosis & Hemostasis Reference Ranges (updated 1/11/2021), Indiana University Health Pathology Laboratory (2021)
  4. Critical Values List (CORP-APPEND-0104A, Rev. 46), ARUP Laboratories (2026)
  5. Prothrombin Time Test and INR (PT/INR), MedlinePlus, National Library of Medicine
  6. Prothrombin Time [PT], Practical-Haemostasis.com
  7. Pre-Analytical Variables, Practical-Haemostasis.com
  8. ISI and INR, Practical-Haemostasis.com

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

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