Skip to content

Protein S

Published: July 27, 2026Updated: August 25, 2026

Protein S is a vitamin K-dependent anticoagulant protein that acts as a cofactor for activated protein C, which helps in regulating blood coagulation.

Quick summary

Protein S is essential for inhibiting blood clot formation.

Medically reviewed by WellAlly Editorial TeamLast reviewed August 25, 2026Editorial policy

Key facts at a glance

Units
%
Clinical utility
diagnostic
Result kind
semi_quantitative
FDA-cleared
Roche Protein S, Siemens Protein S, Stago Protein S
interpretation: {"text":"Levels below 65% may indicate a Protein S deficiency, increasing the risk of thrombosis.","label":"Low"}; {"text":"Levels above 140% are generally not associated with increased risk but may warrant further investigation.","label":"High"}

Protein S is a natural anticoagulant that plays a crucial role in the regulation of blood coagulation. It functions as a cofactor for activated protein C, which inactivates clotting factors Va and VIIIa, thereby preventing excessive clot formation.

A deficiency in Protein S can lead to an increased risk of thrombosis, as the balance of the coagulation system is disrupted. This can result in conditions such as deep vein thrombosis (DVT) or pulmonary embolism (PE).

The measurement of Protein S levels is important in the diagnosis of thrombophilia, a condition characterized by an increased tendency to form blood clots. Levels below the normal range may indicate a Protein S deficiency.

Frequently asked questions

What causes low Protein S levels?
Low Protein S levels can be caused by genetic deficiencies, liver disease, vitamin K deficiency, or certain medications.
How is Protein S deficiency treated?
Treatment typically involves anticoagulant medications to prevent blood clots and addressing any underlying conditions.
  1. 1
    Protein S Deficiency - National Hemophilia FoundationSource
  2. 2
    Role of Protein S in Coagulation - PubMedSource

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 25, 2026. · Read our editorial policy