S100 (S100B)
A protein used as a blood marker in melanoma, the most serious form of skin cancer. It is not a screening test — its main role is to help monitor an advanced or treated melanoma, where a rising level can signal that the cancer is progressing or has come back.
What it measures
S100 (usually the S100B form) is a protein that can be released into the blood by melanoma cells, particularly in more advanced disease. It is used as a tumour marker specifically in melanoma, the most serious skin cancer, which arises from the pigment-producing cells of the skin. Importantly, it is not a way to screen for or diagnose melanoma — early melanoma is found by looking at the skin and by removing and examining a suspicious mole, not by a blood test, and S100 is usually normal in early disease. Its value is in follow-up of melanoma that is more advanced or that has spread: a raised or rising S100B level tends to reflect a larger burden of melanoma and can give an early signal that the disease is progressing or recurring, prompting scans. It is often used together with LDH, another marker of disease activity in advanced melanoma. Because S100 can also rise in some non-cancer situations (it is also found in the brain and is used in head-injury research), it is interpreted only within the specialist care of a known melanoma.
What a high value can mean
- Advanced or spreading melanoma — a higher level tends to reflect a larger amount of melanoma.
- A rising trend during follow-up — can signal progression or recurrence, prompting imaging.
- Non-melanoma causes — S100 is also present in the brain and can rise after head injury or in some other conditions, so it is read in context.
What a low value can mean
- A normal or low level is expected in early or well-controlled melanoma — and reassuring, but does not by itself prove the cancer is gone.
- A falling level during treatment — suggests the melanoma is responding.
When to discuss with a doctor
S100 is a specialist test, used within the care of a known melanoma — especially advanced or treated disease — rather than to screen for skin cancer. A suspicious or changing mole is assessed by looking at the skin and, if needed, removing it for examination, not by this blood test. Where S100 is used, its value is in the trend during follow-up: a rising level can be an early prompt to look for progression or recurrence with imaging, always interpreted with LDH, examination and scans by the oncology or dermatology team. A single mildly raised value has non-melanoma explanations and is not alarming on its own. Mediora.AI shows S100 alongside LDH so the pattern is visible; interpreting it belongs with your specialist team.