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Protein S Free (Anticoagulant Activity) in Plasma

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

This test measures the amount of free Protein S in your plasma, a protein that helps prevent abnormal blood clots. Protein S works with other blood proteins to maintain healthy circulation.

Quick summary

["Free Protein S supports blood clot prevention and helps maintain healthy circulation."]

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

Key facts at a glance

LOINC
59224-6
Short code
PSA Free Prt-mCnc
Synonyms
PSA Free Prt-mCnc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
0
Ref high
4
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
84153
SNOMED
39392002
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 Might Be Ordered

Your healthcare provider may recommend this test if you have a history of blood clots, certain blood disorders, or conditions that affect protein function. It may also be used to monitor treatment for inherited or acquired clotting risks.

What to Expect Before the Test

No special preparation is needed. You may have a small needle inserted into a vein (usually in your arm) to draw a blood sample. If you’re on blood thinners, let your provider know before the test.

How the Test Works

Blood is collected and processed to separate plasma. The test measures the activity of free Protein S using a specialized immunoassay method to detect its anticoagulant properties.

What the Results Mean

Results help determine if your free Protein S levels are within expected ranges. Low levels may indicate an increased risk of clotting, while normal levels suggest balanced blood coagulation function.

Reference range

Normal: <4.0 ng/mLN/A

Specimen: serum

No special preparation required. For PSA, avoid sexual activity for 24-48 hours before the test.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Protein S Free

Frequently asked questions

What causes low free Protein S levels?
Low free Protein S can result from inherited genetic mutations, liver disease, pregnancy, oral contraceptives, or certain medications that affect protein synthesis or function.
How is this test different from total Protein S?
This test specifically measures *free* Protein S (unbound to other proteins), while total Protein S includes both free and bound forms. Free Protein S is the active form that prevents clotting.
Can diet or lifestyle affect Protein S levels?
Dietary factors like vitamin K intake (found in leafy greens, nuts) may influence Protein S levels, but lifestyle changes alone won’t alter test results. Follow your provider’s guidance for any dietary restrictions.
  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