S100B is a type of protein found in neurons, as well as other cells outside of the brain. Its role seems to depend on its concentration. Low levels of S100B are found in healthy brains, indicating a supportive role when in low quantity. Levels of S100B rise considerably after brain injury, and may involve a toxic effect to the brain (this is still being studied, however).
A recent study looked at the predictive value of S100B, specifically to discover if the biomarker could be useful in deciding whether or not to use a CT scan in cases of mild traumatic brain injury. The CT scan often fails to show damage in mild cases of brain injury, and can be a costly and potentially dangerous (exposure to radiation) test. Therefore a reliable biomarker could help in the decision process.
The study found that low levels of S100B accurately predicted normal CT scans, implying that low levels could support a decision to not use a CT scan. However, the S100B sample should be drawn within 3 hours after injury and should also be taken into consideration with other symptoms before making a decision to use a CT scan or not.
Unden J, & Romner B. Can low serum levels of S100B predict normal CT findings after minor head injury in adults? An evidence-based review and meta-analysis. Journal of Head Trauma Rehabilitation. (July 2010).