S100 test (S100B): what it shows and how the result is read

S100 is a member of the family of calcium-binding proteins, found in large amounts in melanocytes (the cells of the skin that produce pigment) and in the nervous system. Some forms of the protein in this family, especially S100B, get into the blood in increased amounts when there are more melanoma cells or when they are actively growing, so S100 is used as an additional tumour marker in people with melanoma. According to MedlinePlus, tumour markers in general are most often ordered not to make the initial diagnosis, but after the diagnosis has already been confirmed by other tests (biopsy, imaging tests). They are used to follow whether the treatment is working, whether the disease has come back after treatment, and sometimes to assess the stage of the disease. Importantly, a tumour marker result is never assessed on its own: a raised level can also be caused by conditions unrelated to cancer, so it is always interpreted together with imaging tests, the clinical picture and the doctor's assessment.

• When following the course of treatment of an already diagnosed melanoma and assessing whether the treatment being given is working
• After treatment for melanoma — looking for early signs that the disease is coming back
• When assessing the activity of widespread (metastatic) melanoma during monitoring
• Together with imaging tests, when a known oncological disease is being monitored
• After a head injury, go to an emergency department — S100B is not a standalone way to decide whether a CT scan is needed. In some countries (Sweden, for example) it is used within 6 hours of a mild head injury as an additional test that helps to select patients who may not need a CT scan (according to the Unilabs laboratory description); the UK NICE 2023 head injury guideline does not include it in its recommendations

There is no single, universally agreed S100 reference interval — the limits depend on the laboratory method, so look first at your own result sheet. For example, the Swedish Unilabs laboratory and the Södra Älvsborg Hospital laboratory give an S100B limit of below 0.11 µg/l for adults; for children (under 18) the Södra Älvsborg laboratory gives no reference interval.

A raised S100: what it can mean

In people with melanoma, a raised S100 is most often associated with a larger amount of tumour tissue or with the activity of the disease, but a tumour marker result is never assessed on its own — it can also be affected by conditions unrelated to cancer. The result is always interpreted by a doctor, together with imaging tests and the clinical picture.

What can raise S100

• Progression of the melanoma or a larger amount of tumour tissue in the body
• The disease coming back after treatment
• Some conditions unrelated to cancer that can raise the level of tumour markers
• Brain injury — a mild head injury (concussion), stroke, brain tumours, brain damage after cardiac arrest (according to the Unilabs laboratory description)

What to do next if S100 is raised

• Discuss the result with your doctor together with imaging tests (for example, ultrasound, CT or MRI)
• Consider whether the test needs to be repeated, so that the trend in the level over time can be seen
• The doctor may adjust the monitoring or treatment plan according to the overall clinical picture

A falling S100 level during monitoring is usually considered a good sign — tumour marker tests are often used precisely so that it can be seen whether the treatment is working. Even so, the value on its own, without context, does not reflect the whole picture, so the trend over time is interpreted by a doctor.

Why the level can fall

• A response to the melanoma treatment being given
• Part of the natural fluctuation of the level between tests, unrelated to any change in the disease

What to do next if the level is falling

• Discuss with your doctor whether the falling trend matches the results of imaging tests
• Keep to the schedule of monitoring tests that has been set

Who assesses the result

According to MedlinePlus, tumour marker tests are usually intended not to make a diagnosis, but to follow how well treatment is working, because conditions unrelated to cancer can also raise the level. A tumour marker result is therefore never assessed on its own: it is always interpreted together with imaging tests, the clinical picture and the doctor's assessment.

Related pages: CEA, AFP, CYFRA 21-1, CA 19-9, CA 72-4, how to prepare for a blood test and how to read blood test results.