Protein S is another protein in the blood that works like a natural anticoagulant. It was discovered in Seattle, hence the name Protein S. Like Protein C and many of the clotting proteins it requires vitamin K for normal activity. It is present in the blood in two forms either bound to another protein called C4b-binding protein or as a free protein. Protein S works as a “co-factor” for protein C. This means that protein S helps to speeds up the action of protein C, and in turn helps to keep the blood liquid in your circulation.
Protein S deficiency
Protein S deficiency can be inherited or acquired; the latter is usually due to liver disease. It is a dominant condition (inherited from one parent) and people with the deficiency usually have a protein S level around 50% of normal. The deficiency increases your risk of venous thrombosis and pulmonary emboli. There have been some studies suggesting that protein S deficiency is associated with arterial disease, like heart attacks and strokes, but the link is very weak, so it is not thought to be a major risk for these conditions. Protein S deficiency is a rare condition and is found in about 1 in 700 of the normal population in Europe and the USA. It is found in about 5% of people who have a spontaneous thrombosis. However this condition is far more common in Japan and it is found in about 13% of people with a history of a blood clot.
What does it mean if I have Protein S deficiency?
People with this condition have a higher risk of both DVT and PE. It is thought that about 60% of people with this condition will have a blood clot at some time in their life. They usually occur around the age of 40 to 45yrs. People with this condition will usually remain on anticoagulants longterm after their first clot.
Pregnancy and the Pill
Protein S is strongly affected by female hormones. The protein S level will be low in women on an oestrogen containing contraceptive, so it is important that you are not taking the pill when you have a blood test for this condition. Protein S levels also fall during pregnancy and may be as low as 30% of the level seen in a non-pregnant woman. As this happens in all women it does not appear to be linked to thrombosis in pregnancy, in fact the low protein S level may have an important physiological role to prevent bleeding. The diagnosis of protein S deficiency cannot be made in pregnancy. There is possibly a link between protein S deficiency and recurrent miscarriage, but it is not a strong correlation.