Protein S (Free) in Plasma
This test measures the level of free Protein S, a natural blood protein that helps prevent blood clots by regulating another blood-clotting protein called Protein C. Low levels may increase your risk of clotting.
["Free Protein S helps maintain healthy blood flow by regulating clotting factors."]
Key facts at a glance
- LOINC
- 105767-8
- Short code
- Proteus sp DNA Ct Bld Pos Qn NAA+probe
- Synonyms
- Proteus sp DNA Ct Bld Pos Qn NAA+probe
- Category
- chemistry
- Sample
- whole-blood
- Container
- Lavender-top (EDTA)
- Units
- N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- probe-amp-tar
- Specimen volume
- 3 mL
- Stability
- Room temperature: 4h, Refrigerated: 24h
- Hemolysis max
- 0
- Lipemia max
- 0
- Icterus max
- 0
- TAT
- 4–24 hours
- Order
- routine
Why This Test Is Done
Your healthcare provider may order this test if you have a history of blood clots, certain blood disorders, or conditions that affect how your blood clots, such as antiphospholipid syndrome or protein C or S deficiency. It may also be done as part of routine screening in certain populations or before surgeries that increase clotting risk.
What to Expect Before the Test
You may be asked to fast for 8–12 hours before the test, but this isn’t always required. Bring any medications you’re taking to your appointment. Avoid smoking or consuming alcohol for a few hours beforehand if instructed.
How the Test Is Performed
A blood sample is drawn from a vein in your arm by a healthcare professional. You may feel a small pinch, but it usually doesn’t cause discomfort afterward.
What the Results Mean
Results are typically reported as a ratio or percentage compared to a standard. Low levels of free Protein S may indicate an increased risk of clotting, while normal levels suggest healthy blood-clotting regulation. Your provider will discuss these results with you in the context of your overall health.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Low free Protein S levels are associated with an elevated risk of venous thromboembolism (blood clots in veins) or arterial clots (e.g., heart attacks or strokes). It’s particularly important for patients with inherited or acquired clotting disorders. Regular monitoring may be recommended if you have a history of clotting events or are at high risk.
Short name
Free Protein S Level
Frequently asked questions
What does a low or high free Protein S level mean?
Does fasting affect this test?
How often should I get this test?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
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.
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