Skip to content
Blood Test Tracker

Factor VIII Activity

Measures how well factor VIII, a clotting protein that is low in hemophilia A, is working in your blood.

AI-verified against cited sources. Awaiting expert reviewAlso called FVIII, FVIII:C, F8LOINC 3209-4

Check your result

Sex (optional)

Type your result to see where it falls.

Nothing you type here leaves your browser.

What it is

Factor VIII is a protein your body uses to form blood clots. It travels in the blood attached to another protein called von Willebrand factor. Labs report its activity as a percentage of normal or in IU/dL, and the two numbers are usually read as the same (100% is about 100 IU/dL). Clinicians use it to look into hemophilia A and von Willebrand disease and to follow treatment with factor VIII products.

Why it matters

Low factor VIII can cause longer bleeding after injuries, surgery or dental work, and in severe hemophilia A, bleeding into joints and muscles. High factor VIII is linked to a higher chance of vein clots, but levels move so much with inflammation, stress and pregnancy that guidelines do not recommend routine testing for it.

If it’s low

A low factor VIII means the blood may clot more slowly. The best-known cause is hemophilia A, an inherited condition that mostly affects males. Von Willebrand disease also lowers it, because von Willebrand factor protects factor VIII in the blood. Rarely, the immune system makes antibodies against factor VIII (acquired hemophilia). Slightly low results are also common in people with blood group O.

Common causes

  • Blood group O, which goes with somewhat lower von Willebrand factor and factor VIII
  • A sample that was handled or stored incorrectly, such as refrigerated or processed late
  • Test interference from lupus anticoagulant or certain blood thinners on the one-stage clotting assay
  • Von Willebrand disease
  • Hemophilia A (inherited, mostly in males)
  • Acquired hemophilia A, caused by antibodies against factor VIII (rare)

What can help

  • Ask which follow-up tests make sense, such as von Willebrand tests, a repeat factor VIII, a second (chromogenic) method or an inhibitor screen
  • Ask whether a hematologist or a hemophilia treatment centre should be involved
  • Mention a low result to anyone planning surgery or dental work on you

If it’s high

A higher-than-usual factor VIII is common and often temporary. It rises with inflammation, stress, hard exercise and pregnancy. Levels at the high end have been linked to a higher chance of vein clots, and in people who already had a clot, levels above 200 IU/dL (about 200%) were linked to more repeat clots. Because levels vary so much over time, guidelines do not recommend routine testing for it.

Common causes

  • Pregnancy
  • Hard exercise or stress around the time of the blood draw
  • Infection, inflammation or other illness, because factor VIII is an acute-phase protein
  • A blood group other than O
  • A recent factor VIII infusion, in people who receive treatment
  • An inherited tendency to higher levels

What can help

  • Ask whether the test should be repeated when you are rested and well, since stress and inflammation raise factor VIII
  • If you have had a blood clot, ask whether a high level changes anything in your care, since guidelines do not recommend routine testing for it

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: 55–200 %. Source: Mayo Clinic Laboratories (Extended Catalog), ARUP Laboratories, World Federation of Hemophilia (Haemophilia), British Journal of Haematology. Labs treat below 5 % as urgent.

WFH hemophilia A severity by baseline factor VIII level (for people with hemophilia A)

  • Severe range (under 1%)below 1 %
  • Moderate range (1% to under 5%)1 to under 5 %
  • Mild range (5% to under 40%)5 to under 40 %
  • Above the hemophilia A range (40% or higher)40 and above %

What can affect your result

  • Strenuous exercise, stress, inflammation and pregnancy, which raise it
  • Blood group: people with blood group O tend to have lower levels
  • The assay: one-stage clotting and chromogenic methods can differ, especially in mild hemophilia A
  • Factor VIII infusions, and other treatments for hemophilia
  • Lupus anticoagulant and some blood thinners, which can interfere with the one-stage assay
  • How the sample is processed and stored
  • Strenuous exercise, stress, inflammation and pregnancy raise factor VIII for a while, which can hide a mild deficiency. Rest for a few minutes before the draw and avoid hard exercise first.
  • Factor VIII infusions raise the level, so for diagnosis the sample should be drawn before replacement therapy.
  • Lupus anticoagulant, some blood thinners and factor inhibitors can interfere with the usual one-stage clotting assay.
  • One-stage and chromogenic assays can disagree, especially in mild hemophilia A, so both may be run.
  • Samples that are refrigerated or processed late can lose factor VIII.

A snapshot of the level at the time of the draw. Stress, illness or pregnancy can raise it temporarily.

When to talk to a clinician

Get urgent medical help for bleeding that will not stop, blood in your urine or stool, a head injury or severe headache, or a painful, swollen joint or muscle after a minor knock. ARUP's list of critical results includes factor VIII activity of 5% or lower, so a lab may call you or your clinician about a result that low.

Questions you could ask

  • Which method did my lab use, and does that matter for my results?
  • Was my blood drawn when I was stressed, ill or pregnant, and should we repeat the test?
  • Does a high level matter for my history of blood clots or my family history?
  • What could explain a low factor VIII in my case, such as von Willebrand disease, blood group O or hemophilia A, and what test comes next?
  • Should the test be repeated or run with a second (chromogenic) method?
  • Should I see a hematologist or a hemophilia treatment centre?

Questions people ask

Is a low factor VIII always hemophilia?

No. Von Willebrand disease, blood group O, an antibody that interferes with the test, and sample handling can all give a lower result. Clinicians look at symptoms, family history and other tests, such as von Willebrand factor, to sort out the cause.

Why do the percent and IU/dL numbers look the same?

Factor VIII is measured against a standard where normal pooled plasma is set at 100%, which is about 100 IU/dL or 1 IU/mL. So 40% is about 40 IU/dL or 0.40 IU/mL. The World Federation of Hemophilia notes that the two are not strictly interchangeable, so compare your result with the unit and range printed on your own report.

Why might two kinds of factor VIII tests be ordered?

Some people with mild hemophilia A have a normal result on the usual one-stage clotting test but a low result on the chromogenic test. The World Federation of Hemophilia recommends using both when looking into possible hemophilia A.

Show the science and sources

Units

  • 1 % = 1 %
  • 1 % = 1 IU/dL
  • 1 % = 0.01 IU/mL

Open the Factor VIII unit converter

Is a change real?

Factor VIII naturally varies about 8.7% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 27% 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

Factor VIII is a cofactor for factor IXa in the tenase complex, circulates bound to von Willebrand factor, and is an acute-phase reactant. Activity is measured by a one-stage aPTT-based assay using factor VIII-deficient plasma or by a chromogenic assay. Some mild hemophilia A cases read normal on the one-stage assay but low on chromogenic and two-stage assays, and the reverse also occurs, so the WFH 2020 guideline recommends running both in the initial diagnostic work-up. Lupus anticoagulants, specific inhibitors and some anticoagulants can interfere with one-stage assays. Results should be traceable to a WHO international standard and reported in IU/mL or IU/dL; a percentage is appropriate mainly when the reference is a pooled normal plasma that is not traceable. WFH Table 2-1 classes hemophilia as severe (under 1 IU/dL), moderate (1–5 IU/dL) or mild (5 to under 40 IU/dL) by baseline level. Factor VIII and VWF rise with strenuous exercise, stress, inflammation and pregnancy, and non-O ABO alleles raise VWF and secondarily factor VIII. In the MEGA study the odds ratio for VTE with factor VIII above the 95th centile was 16.0 after adjustment for other factors. Among people with a first VTE, a level above 200 IU/dL carried a hazard ratio of 3.4 for recurrence versus 100 IU/dL or less. Because of acquired modifiers and variability over time, the BSH 2022 thrombophilia guideline does not recommend routine testing for raised factor VIII, and ARUP does not recommend factor VIII when screening for thrombophilia. Mayo notes that this test is not useful for detecting hemophilia carriers.

LOINC 3209-4 (primary), 3208-6, 49865-9, 3210-2. Specimen: plasma.

Sources

  1. von Willebrand Disease Profile, Plasma (test AVWPR), including Coagulation Factor VIII Activity, Mayo Clinic Laboratories (Extended Catalog)
  2. Chromogenic Factor VIII Activity Assay, Plasma (test CHF8), Mayo Clinic Laboratories (Extended Catalog)
  3. Factor VIII, Activity (ARUP test 0030095), ARUP Laboratories
  4. Critical Values List (CORP-APPEND-0104A, Rev. 46, April 2026), ARUP Laboratories (2026)
  5. WFH Guidelines for the Management of Hemophilia, 3rd edition: Chapter 2, Comprehensive Care of Hemophilia (Table 2-1), World Federation of Hemophilia (Haemophilia) (2020)
  6. WFH Guidelines for the Management of Hemophilia, 3rd edition: Chapter 3, Laboratory Diagnosis and Monitoring, World Federation of Hemophilia (Haemophilia) (2020)
  7. Thrombophilia testing: A British Society for Haematology guideline, British Journal of Haematology (2022)

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

Related markers