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Doctor's Assessment Included

Every result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

Coagulation

Protein S (Activity)

Protein S Activity testing assesses the cofactor system supporting natural anticoagulation. Proactive screening reveals deficiency before a thrombotic event.

What It Measures

This test measures the functional anticoagulant activity of Protein S, reflecting its capacity to support activated Protein C. Results are expressed as a percentage of normal.

Why It Matters

Protein S deficiency is an underdiagnosed cause of hereditary thrombophilia. Identifying deficiency enables appropriate risk management.

When to Test

Testing can be indicated for unexplained venous thrombosis, family history of thrombophilia, or as part of comprehensive thrombophilia screening. Avoid testing during pregnancy or on warfarin.

Symptoms

Low Levels

Low Protein S activity may be associated with increased thrombosis risk. Symptoms of thrombotic events include leg pain and swelling (DVT), sudden shortness of breath (PE), or stroke-like symptoms.

High Levels

Elevated Protein S activity is not clinically significant and does not require intervention.

Lifestyle Tips

If Protein S deficiency is confirmed, reduce modifiable thrombosis risk factors. Oestrogen-containing contraceptives can further reduce Protein S — always discuss with your doctor.

Frequently Asked Questions

Why do pregnancy and contraceptives affect Protein S?
Oestrogen increases C4b-binding protein, which binds and inactivates free Protein S. Pregnancy can reduce free Protein S by up to 50%.
Can Protein S be tested on warfarin?
No. Warfarin suppresses Protein S, giving falsely low results. Test at least 2 weeks after stopping warfarin.
How is Protein S deficiency classified?
Three types: Type I (low total and free), Type II (normal levels but reduced function), Type III (normal total but reduced free).