Protein S test
Quick answer: A protein S test measures protein S, a natural substance in blood that works with protein C to prevent unwanted clots. It is ordered after unexplained or repeated blood clots, or when there is a family history of protein S deficiency.
- SampleBlood
- TakesAbout 5 minutes
- Before the testYour doctor will choose the timing, as recent clots, pregnancy, hormone pills and blood thinners can affect results; do not stop any medicine on your own.
- In fertility careProtein S levels naturally fall in pregnancy, so results taken during pregnancy need careful interpretation by your doctor.
How the test is done
- 1Your doctor plans the timing, as pregnancy, hormone pills and blood thinners affect results
- 2Blood is drawn from a vein into a blue-top tube
- 3The tube is filled to the right level
- 4The lab measures free or total protein S, or its activity
What your results can mean
Reading your report
Normal ranges differ between labs, so compare your result with the range printed on your report and discuss it with your doctor.
If the result is outside the usual range
A low protein S can be seen with an inherited deficiency, which raises the risk of clots, and also in pregnancy, with hormone pills, liver disease, a recent clot or warfarin. High levels are not usually a concern.
Who usually has this test
People with unexplained or repeated blood clots and close family of people with protein S deficiency.
Risks and aftercare
Apply firm pressure to the needle site to reduce bruising.
Names and options
Also called: protein S activity, free protein S
Listed by the hospital as
- PROTEIN S, PLASMA
Questions about this test
Why is protein S low in pregnancy?
Protein S naturally falls during pregnancy as part of the body's normal changes. This makes results taken during pregnancy hard to interpret, so testing is often done before or a few months after pregnancy.
Do birth control pills affect protein S?
Yes. Oestrogen-containing pills and hormone treatment can lower protein S. Tell your doctor about any hormone medicines, but do not stop them on your own.
What is free protein S?
Only part of protein S in the blood is free and active. Many labs measure free protein S because it more closely reflects how much is available to prevent clots.
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Tamara Hospital & IVF Centre, Rajajinagar
34/3, 10th Cross Road, opp. Vardhaka Sardar Patel School, 1st N Block, 2nd Stage, Rajajinagar, Bengaluru 560010
The hospital is open 24 hours. Call before you come to check the timing for your scan or sample collection.
Get directions ↗IVF Pulse fertility care at Tamara Hospital →
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This page explains each test in general terms so you know what to expect. It is not medical advice: your doctor decides which tests you need and what your results mean. Reference ranges differ between laboratories. Updated 4 November 2026.