Protein S

Many laboratories print a lower range for women (around 55 to 125 %) than men (around 65 to 140 %), and free, total and activity assays each have their own range; use the range your lab printed.

Protein S is a vitamin K dependent protein that helps activated protein C to switch off clotting, so it is another of the body's natural brakes on clotting. Some of it circulates free and some is bound to another protein; only the free part is active, which is why laboratories often measure free protein S. The result is a percentage of pooled normal plasma.

Doctors request it as part of a thrombophilia screen after an unexplained clot, particularly in a young person or with a strong family history. It is one of the more difficult clotting proteins to measure reliably, so a single low value is usually repeated.

• There are no common causes of a high protein S and a high value is not of any importance

• Pregnancy and oestrogen-containing medicines, including the combined pill and HRT
• Warfarin and other vitamin K blocking medicines
• Inherited protein S deficiency, which is uncommon
• Liver disease
• Low vitamin K
• A recent clot, surgery, infection or severe illness
• Nephrotic syndrome
• Some direct oral anticoagulants interfere with some methods

• It falls considerably in pregnancy and on oestrogen-containing medicines as an expected effect, so a low value in these settings is very common
• A level measured on warfarin or shortly after a clot reads low and is repeated later
• Women sit lower than men in the general population
• Free, total and activity methods give different numbers and have different ranges

No fasting needed. Citrate tube. Best measured off warfarin and oestrogen-containing medicines, not in pregnancy, and some weeks after any clot.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.