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Protein S (Free) Activity

Published: August 9, 2026Updated: August 16, 2026

This test measures the activity level of free Protein S, a natural blood protein that helps prevent abnormal blood clots.

Quick summary

["Protein S supports blood clot prevention; low levels may increase clot risk."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
49640-6
Short code
Prot S Ag/Fact X Ag PPP
Synonyms
Prot S Ag/Fact X Ag PPP
Category
chemistry
Sample
plasma
Container
Lavender-top (EDTA) or Green-top (Heparin)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
coag
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 blood clots, certain blood disorders, or conditions like antiphospholipid syndrome. It can also be part of screening for inherited clotting disorders or monitoring treatment for conditions affecting blood clotting.

What to Expect Before the Test

No special preparation is needed. You may have a blood draw from a vein in your arm. If you’re on blood-thinning medication, check with your doctor first, as it may affect results.

What the Test Results Mean

Results are typically reported as a percentage or ratio relative to a normal range. Low levels may indicate an increased risk of clots, while normal levels suggest healthy blood clot regulation.

Common Conditions Monitored

Protein S deficiency is linked to inherited clotting disorders. Your doctor may use this test to diagnose or monitor conditions like thrombophilia or to guide treatment decisions for patients on anticoagulant therapy.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance: Protein S Deficiency and Clotting Risks

Short name

Free Protein S Activity

Frequently asked questions

How long does it take to get my Protein S test results?
Results are usually available within 1 to 3 business days, depending on your lab. Your doctor will discuss them with you during a follow-up visit.
Can I eat or drink before this test?
You can eat and drink normally before the test, as no fasting is required. However, inform your doctor if you’re on blood-thinning medications, as they may affect results.
What are the normal reference ranges for Protein S?
Reference ranges vary by lab, but typically free Protein S activity is considered normal at levels above 30% of a reference value. Your doctor will interpret results in the context of your overall health and medical history.
  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 16, 2026. · Read our editorial policy