S100 is a calcium-binding protein found mainly in nerve tissue cells and skin cells (keratinocytes).

 It got its name from the first mention, as a soluble protein in a 100% solution of ammonium sulfate. Like all proteins, it performs a number of vital functions: structural, transport, contractile, etc. (see "total protein"). Therefore, protein is present in small amounts in the blood of any healthy person.

There are two main reasons for the pathological increase in the concentration of S100 in the blood:

• Massive death of nerve tissue cells and the release of S100 protein into the bloodstream in large quantities;
• The presence of melanoma skin cancer in the body, which intensively produces S100.

This connection between the nervous system and the skin is explained by the fact that during the embryonic period these tissues develop from a single ectodermal germ.

Based on the reasons for elevated S100, the clinical significance of this assay lies in the diagnosis of melanoma and CNS diseases characterized by nerve cell death (stroke, traumatic brain injury, Creutzfeldt-Jakob disease, neurodegeneration).

There may be a slight increase in protein (up to 0.4 µg/L) S100 in some diseases of the lungs, gastrointestinal tract, genitourinary sphere. Increases up to 2.0 µg/L are seen in very severe bacterial infection.

In grade I melanoma, the amount of S100 in the blood is usually within norm limits, while grade II - III melanoma is detected by this oncomarker in 4-20% of cases.

At grade IV, the level of S100 in the blood is increased in 30-90% of cases. By itself, the detection of S100 in the blood cannot be a reason for a diagnosis. The result of the test directs the doctor to further diagnostic tactics.

The great value of the test is the possibility of monitoring the effectiveness of treatment and detecting a recurrence of the disease. In these cases, the concentration of S100 will vary in comparison with the level before the start of treatment.

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