What Is A Protein S Deficiency? Issue, Functions And Treatment

A protein S deficiency is a genetic trait, although this type of deficiency can sometimes be acquired. Protein S plays an important role in regulating and controlling blood clotting. A protein S deficiency increases the likelihood of a blood clot forming in the body, or forming in an unregulated manner.

In other words, there is a certain amount of danger associated with having a protein S deficiency. A person with the deficiency has a predisposition towards thrombosis in the veins, particularly in the deep veins of the arms and legs.

More Good News Than Bad

There is a mix of good and bad news associated with protein S deficiency. About 1 out of every 500 people will have the deficiency, but they will almost always have it in a mild form. Only a few will ever experience a clotting problem that can be directly traced to the deficiency.

If a person having a mild form of the deficiency suffers from a blood clot, the deficiency may or may not have been a contributing factor. Even if it has, it is not always easy to tell how much of a factor it is. An additional bit of good news is that the number of cases of severe protein S deficiency is very rare.

A person with a severe deficiency would be at greater risk of uncontrollable blood clotting, but the number of such people who have actually have had a problem is so small that no reliable statistics are available.

Arterial Thrombosis Not An Issue

There is one other piece of good news. Most of us have heard of arterial thrombosis, a blood clot in an artery. An arterial thrombosis is very often bad news.

The good news is that there does not seem to be a meaningful correlation between having a protein S deficiency, mild or severe, and experiencing a blood clot in an artery. When protein S deficiency does cause a clotting problem, it is almost always in a vein and never in an artery.

It is usually the deep veins in the arms or legs that tend to suffer from the clots. About 1 in every 1,000 people will have a deep vein clot at one time or another. The deficiency contributes to between 1% and 5% of the clotting events. By looking at the statistics, you can see why protein S deficiency is not a household word, yet the term blood clot is.

Put another way, protein S deficiency is quite rare in a healthy population. The word thrombosis – blood clot – comes from thrombin, an enzyme that causes blood to coagulate.

What Is Protein S, And How It Functions

The “S” stands for Seattle, as that is where the protein was first discovered. Protein S is an anticoagulant protein. There are two types of protein S:

  • A free form protein
  • A bound protein

Only the free form protein is of interest here. It is a co-factor in preventing uncontrolled coagulation in the blood. It works together with another protein, protein C, to regulate clotting.

Protein C actually does most of the work, but protein S functions as a helper, an important helper at that. The two proteins function to cover the platelets in the blood with a substance that controls coagulation. If protein S is deficient, protein C cannot do its job effectively.

The Three Types Of Deficiency

There are three types of protein S deficiency:

  • A Type I deficiency exists when the amount of protein S, both free form and bound, is inadequate.
  • A Type II deficiency means that the amount of protein S may be adequate, but the molecules of protein S are in some way defective.
  • A Type III deficiency means that the total amount of protein S is adequate, but there is a shortage of the free-form proteins.

The different types are, of course, meaningful to medical practitioners, but to the patient it simply boils down to not having an adequate amount of free-form protein S in the system.


Treatment of a deep venous blood clot usually involves the long-term use of an anti-coagulant such as warfarin or heparin. The dosage is prescribed on a case-by-case basis as there is no “one size fits all” when bleeding or clotting is the problem.

Treatment for clotting events can last for several months, but can be much longer when a patient is determined to be deficient in protein S. How much longer treatment should continue would depend upon how severe the deficiency is. A period of treatment lasting 6 to 9 months is not uncommon.

If a patient constantly faces a high risk of a life-threatening thrombosis event, prescribed medication may be required throughout the person’s lifetime. As a precaution, or as a preventive measure, those having the deficiency are sometimes advised to restrict the amount of vitamin-K rich foods in their diet.

Those who have experienced a blood clot and are on anti-coagulant medication are usually advised to avoid vigorous sporting activities where contact is involved.

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