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Unfractionated Factor VIII Activity (Plasma)

Published: August 9, 2026Updated: August 17, 2026

This test measures the activity level of Factor VIII, a protein in your blood that helps prevent excessive bleeding. It’s often used to monitor treatment for bleeding disorders or hemophilia.

Quick summary

["Factor VIII helps control bleeding; low levels may indicate a bleeding disorder or need for treatment monitoring."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 17, 2026Editorial policy

Key facts at a glance

LOINC
40739-5
Short code
OF Unincubated NFr RBC
Synonyms
OF Unincubated NFr RBC
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
4.2
Ref high
5.9
Critical range
2 N/A – 7 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
85041
SNOMED
900000000000003001
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

Why This Test Is Done

Your doctor may order this test if you have a history of bleeding problems, hemophilia, or other clotting disorders. It’s also used to check how well Factor VIII replacement therapy is working.

What to Expect Before the Test

No special preparation is needed. You’ll provide a blood sample, usually drawn from your arm. If you’re on Factor VIII therapy, your doctor may ask to monitor your levels regularly.

What the Results Mean

Results are expressed as a percentage of normal activity. A lower than normal result suggests bleeding risk or a clotting disorder. Your doctor will interpret results based on your medical history and symptoms.

Common Conditions Monitored

This test helps diagnose or manage bleeding disorders like hemophilia A, von Willebrand disease, or other inherited clotting deficiencies. It’s also used in patients on Factor VIII replacement therapy.

Reference range

Adult males: 4.2-5.9; Females: 3.8-5.1 x 10^12/LN/A

Specimen: serum

No special preparation required. Stay well hydrated.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Factor VIII Activity

Frequently asked questions

What causes Factor VIII levels to be low?
Low Factor VIII levels can result from inherited bleeding disorders (e.g., hemophilia A), liver disease, certain medications, or Factor VIII replacement therapy. Rarely, it may be due to autoimmune conditions or blood clotting disorders.
How often should I get this test?
Your doctor will determine the frequency based on your condition. For patients on Factor VIII therapy, tests may be done weekly, monthly, or as needed to adjust doses and prevent bleeding episodes.
Can diet or supplements affect Factor VIII levels?
Dietary factors like vitamin K or certain supplements (e.g., ginkgo biloba) may indirectly affect clotting factors but are less likely to directly impact Factor VIII levels. Always follow your doctor’s advice regarding supplements.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 17, 2026. · Read our editorial policy