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Blood Test Tracker

Protein C Activity

Measures how well protein C works. Protein C is a natural blood thinner made in the liver that helps keep your blood from clotting too easily.

AI-verified against cited sources. Awaiting expert reviewAlso called PC, Protein CLOINC 27818-4

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What it is

Protein C is a protein made in the liver that needs vitamin K to work. With its partner, protein S, it switches off two of the proteins that make blood clot, so clotting stays in balance. This test measures protein C activity, which is how well it works and not just how much is there. It is usually reported as a percent of normal. Labs use either a color-change (chromogenic) method or a clot-based method.

Why it matters

A low protein C level can raise the chance of blood clots in veins. Most low results come from something other than an inherited problem, such as warfarin, low vitamin K, liver disease, a recent clot or a serious illness. A lasting inherited shortage is uncommon. Testing is usually reserved for a clot with no clear cause, a clot at a young age or in an unusual place, or a strong family history. Results are most reliable when you have recovered from any recent clot and are not taking warfarin.

If it’s low

A low result can be real or temporary. Babies and young children normally have lower levels. In adults, common reasons include warfarin, low vitamin K, liver disease, a recent clot and serious illness. A lasting inherited shortage is less common, and it is confirmed by repeating the test when none of those apply. Even then, a low level raises the chance of a clot but does not mean you will have one.

Common causes

  • Young age, because levels rise through childhood
  • Warfarin or other vitamin K blockers
  • Low vitamin K
  • Liver disease
  • A recent blood clot, an infection or another serious illness
  • Cancer and some cancer drugs, such as L-asparaginase
  • A clot-based test misread because of factor V Leiden or a high factor VIII level
  • Inherited protein C deficiency (uncommon)

What can help

  • Ask when the best time to test is. Labs advise a gap of 2 to 4 weeks after warfarin and waiting until you have recovered from a clot. Never stop a blood thinner on your own.
  • Ask whether a repeat test, a protein C antigen test or genetic testing would help if the level stays low
  • If an inherited shortage is confirmed, ask whether relatives should be offered testing

If it’s high

A result above the range is not known to cause health problems. It can be a lab effect, such as a blood thinner read by a clot-based test or a sample that was activated before testing.

Common causes

  • Natural variation between people
  • Direct oral anticoagulants or heparin, when the lab uses a clot-based test (a lab effect)
  • Warfarin, when the lab uses a chromogenic test, which can read too high
  • A sample that was activated before testing

What can help

  • Usually nothing needs to be done
  • Ask your clinician whether a medicine you take could affect the lab method used

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.

The usual range depends on age. Choose an age to see the one that applies.

Children and teens 11 ranges

See Protein C by age and sex, with the U.S. survey numbers where they exist.

What can affect your result

  • Age: babies and children have lower levels, and the usual range changes until the late teens
  • Warfarin and vitamin K status
  • Liver disease
  • A recent clot, infection or serious illness
  • The test method: chromogenic or clot-based
  • Heparin and direct oral anticoagulants (with clot-based tests)
  • Warfarin and other vitamin K blockers lower protein C. ARUP says not to order the test within 2 to 4 weeks of warfarin. Never stop a blood thinner on your own to get tested.
  • A chromogenic (color-change) test can read too high in people who take warfarin. A clot-based test can read too high with heparin or direct oral anticoagulants, and too low with factor V Leiden or a high factor VIII level.
  • Versiti says the kit maker has shown its chromogenic method is not affected by therapeutic levels of heparin, dabigatran or rivaroxaban. Aprotinin can cause a falsely low result, and a sample activated before testing can read falsely high.
  • Liver disease, vitamin K deficiency, a recent clot, infection and other severe illness lower protein C for reasons that have nothing to do with inheritance.
  • Babies and children have lower levels, so age-specific ranges are needed.

An inherited level does not change over time. Levels linked to warfarin, liver disease or illness can move within days to weeks as the cause changes.

When to talk to a clinician

A low protein C result is not an emergency by itself, but ask the clinician who ordered it to review it with you. Call 911 or your local emergency number for possible signs of a clot: sudden chest pain or shortness of breath, a swollen, painful or warm leg, sudden weakness or numbness on one side, trouble speaking, or a sudden severe headache. If you have low protein C and are starting warfarin, tell your clinician right away about painful dark or purple patches on the skin, which is a rare reaction.

Questions you could ask

  • Which method did my lab use, chromogenic or clot-based?
  • Is protein S being checked at the same time?
  • Could a medicine I take have affected this result?
  • Is this a good time to test, given my recent illness, clot or medicines?
  • Should we repeat the test, or add a protein C antigen test or other clotting tests?
  • Does this result matter for my relatives?

Questions people ask

Why is my lab's range different from the range at another lab?

Protein C ranges depend on the method and the reference group. Published adult intervals include 65–150% at UW Medicine, 74–145% at Indiana University Health and 83–168% at ARUP, and a UK reference text quotes about 65–135 IU/dL. Compare your result with the range printed on your own report.

Can I be tested while on warfarin or soon after a clot?

Warfarin and a recent clot can both lower protein C, so the result may not show your usual level. ARUP says not to order the test within 2 to 4 weeks of warfarin, and UK guidance says to wait until 3 months of treatment for an acute clot. Your clinician decides the timing, and you should not stop a blood thinner on your own.

Is protein C the same as C-reactive protein (CRP)?

No. Despite the similar name, CRP is a marker of inflammation, while protein C is part of the body's natural clot control.

Show the science and sources

Units

  • 1 % = 1 %
  • 1 % = 1 IU/dL

Is a change real?

Protein C naturally varies about 5.5% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 17% 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

Protein C is a vitamin K–dependent serine protease. Thrombin bound to thrombomodulin activates it, and activated protein C with protein S cleaves factors Va and VIIIa. Activity can be measured with a chromogenic assay, in which a snake venom activator (Protac) activates protein C and a color-producing substrate reports the activated enzyme, or with a clot-based aPTT assay. The BSH 2022 guideline lists the chromogenic assay for protein C activity. Clot-based assays can be misread: factor V Leiden (BSH), high factor VIII and hyperlipidaemia (Practical-Haemostasis) can lower results, while heparin and direct oral anticoagulants can raise them (BSH). Chromogenic assays can over-read in vitamin K antagonist therapy and miss rare phospholipid-binding defects. Deficiency is autosomal dominant. Type I (low activity and antigen) accounts for about 75–80% of cases and type II (normal antigen, low activity) for 20–25%. Heterozygous deficiency is reported in about 0.2% of the general population and about 3% of unselected people with venous thromboembolism, and it raises the risk of a first venous clot about 5–7-fold. Mayo's thrombophilia profile follows up protein C activity under 70% (without an acquired cause) with antigen testing. Reference intervals vary by method, and children reach adult levels only after puberty. The BSH advises testing for natural anticoagulant deficiency only after 3 months of anticoagulation for an acute clot, and ASH 2023 suggests testing for protein C, protein S or antithrombin deficiency only in selected situations, such as a family history of the deficiency.

LOINC 27818-4 (primary), 27819-2, 6006-1. Specimen: plasma.

Sources

  1. Protein C, Functional (test 0030113), ARUP Laboratories
  2. Protein C Activity (test code PCCLOT), UW Medicine, Department of Laboratory Medicine and Pathology
  3. Thrombosis & Hemostasis Reference Ranges (updated 1/11/2021), Indiana University Health Pathology Laboratory (2021)
  4. Protein C Assays, Practical-Haemostasis.com
  5. Thrombophilia Profile, Plasma and Whole Blood (test ID AATHR), Mayo Clinic Laboratories (Extended Catalog, via testcatalog.org)
  6. Protein C Activity (order code 1031), Versiti Diagnostic Laboratories
  7. Thrombophilia testing: A British Society for Haematology guideline, British Journal of Haematology (2022)
  8. American Society of Hematology 2023 guidelines for management of venous thromboembolism: thrombophilia testing, Blood Advances (2023)
  9. Protein C and Protein S Tests, 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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