About thrombosis
Thrombophilia: congenital thrombophilia

Protein S deficiency

Protein S is one of the natural anticoagulants found in the blood. People who have insufficient Protein S are more likely to have a venous thrombosis. It is uncertain whether there may also be a very slight risk of arterial thrombosis.

Inheritance of Protein S deficiency
Protein S deficiency is inherited in an autosomal dominant way. This means that if one parent has it, there is a 50:50 chance
(1 in 2) that a child will inherit it.

Other causes of Protein S deficiency
Protein S levels in the blood fall naturally during pregnancy and when women use the combined oral contraceptive pill, or certain types of hormone replacement therapy (HRT).

This is a factor in the increased risk of venous thrombosis in pregnant women, and those taking combined oral contraceptives and HRT.

Occasionally other medical conditions, such as kidney disease, can cause low levels of Protein S.

Protein S deficiency and venous thrombosis
The risk of venous thrombosis is increased in individuals with Protein S deficiency, but the level of risk varies. This is because each family with the condition has a different mutation within the Protein S gene, which affects the level of risk.

One of the best guides to risk is to look at other members of the family with Protein S deficiency. If none if them has had a venous thrombosis, then the risk is probably low unless other risk factors are present. The risk of venous thrombosis increases when one or more additional risk factors are present.

If a number of family members with Protein S deficiency have had a venous thrombosis, the risk is greater. This risk can be reduced through prevention.

Preventing venous thrombosis
People who have Protein S deficiency can reduce the risk of venous thrombosis by making lifestyle changes, such as:

• Leading a physically active life
• Eating a healthy, balanced diet
• Avoiding becoming overweight or obese
• Avoiding long periods of immobility during illness
  or when travelling
• Stopping cigarette smoking

Individuals with Protein S deficiency should also seek medical advice before major surgery, or when they are likely to be immobile for a long period, as this increases the risk of venous thrombosis.

Women with Protein S deficiency should seek medical advice before taking oral contraception or hormone replacement therapy, and when pregnant or planning to become pregnant.

Protein S deficiency and pregnancy
It is important that women with Protein S deficiency who are pregnant discuss this with their obstetrician. As well as an increased risk of venous thrombosis, women with Protein S deficiency may have a slightly higher risk of placental problems during pregnancy. These include having a smaller baby or pre-eclampsia.

Testing for Protein S deficiency
There are a number of tests to detect Protein S deficiency and these may vary between different laboratories. Sometimes, a hospital may send the test to a specialist laboratory.

Usually a test is carried out to measure the amount of Protein S that is working in the blood. This is known as a functional Protein S assay. Other tests may measure the total amount of Protein S present in the blood, or how much free Protein S is present.

Testing other family members
Testing is recommended for close blood relatives of people with a Protein S deficiency, including brothers and sisters and sons and daughters, as there is a 50:50 chance that they will inherit the condition.

Because individuals with congenital thrombophilia do not tend to have problems until they are adults, children are not usually tested until they are old enough to understand why the blood sample is being taken. This is usually after the age of 13.

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