S100B is a calcium-binding protein produced in high abundance within the central nervous system. It is primarily located in glial cells called astrocytes, which support and protect neurons. While its highest concentration is in the brain, S100B is also expressed by other cell types, including melanocytes in the skin, chondrocytes in cartilage, and adipocytes.
The Biological Roles of S100B
The S100B protein has a dual function, operating both inside and outside the cell. Within the cytoplasm of astrocytes, it acts as an internal regulator, participating in cell growth, energy metabolism, and maintaining calcium balance. S100B also interacts with the cell’s structural framework, influencing cell shape and migration.
Astrocytes can also secrete S100B into the surrounding space. In this extracellular environment, the protein acts as a signaling molecule. At low concentrations, it has protective effects on neurons, stimulating their growth and defending them against certain toxic agents. This release allows it to communicate with and influence the behavior of adjacent cells.
S100B as a Clinical Biomarker
A biomarker is a substance that can be objectively measured to provide information about a biological state. S100B is a useful biomarker because its presence in the bloodstream often signals injury to astrocytes in the brain. Under normal circumstances, the blood-brain barrier prevents S100B from entering the blood in significant amounts.
When brain cells are damaged, this barrier can be compromised, causing astrocytes to release S100B into the circulation where it can be detected with a blood test. The protein has a short half-life of about two hours, so sustained high levels point to an ongoing release from the site of injury.
Its stability makes it a reliable substance to measure in a clinical setting. Serum levels remain stable for up to eight hours at room temperature, and the measurement is not significantly affected by the breakdown of red blood cells. This reliability has made it one of the most widely studied blood biomarkers for certain types of tissue damage.
Association with Traumatic Brain Injury
The most established clinical application for measuring S100B is in the management of traumatic brain injury (TBI), especially mild cases like concussions. In emergency departments, clinicians must decide whether a patient with a head injury needs a computed tomography (CT) scan to check for intracranial bleeding. The S100B blood test is used as a screening tool to help make this decision.
The test’s value in this context is its high negative predictive value. A normal S100B level, particularly when a blood sample is taken within three hours of the injury, is a strong indicator that no significant intracranial damage has occurred. If the level is below a specific cutoff (a common threshold is 0.10 µg/L), clinicians can often rule out the need for a CT scan.
This practice reduces unnecessary radiation exposure and healthcare costs. An elevated S100B level does not confirm a brain injury, as the protein is not exclusive to the brain, but it indicates a CT scan is warranted for further investigation.
Connection to Other Medical Conditions
Beyond TBI, elevated S100B levels are associated with other medical conditions. In these cases, the protein’s presence points toward some form of cellular distress or activation, though the specific cause differs.
- Malignant Melanoma: S100B is a prognostic biomarker, as melanoma cells can produce it. Higher blood levels often correlate with a greater tumor load, more advanced disease, and a poorer prognosis.
- Ischemic Stroke: In stroke patients, S100B levels can correlate with the size of the brain infarct and may help predict complications.
- Neurodegenerative Conditions: Levels may be elevated in diseases like Alzheimer’s, where it is associated with the inflammatory processes and astrocyte activation that characterize the condition.
- Psychiatric Disorders: Elevated S100B has been observed in disorders like schizophrenia, where it may indicate astrocyte activation and neuroinflammation rather than widespread cell death.
Measuring and Interpreting S100B Levels
Measuring S100B is performed through a blood test using a serum sample. The test uses an immunoassay, a technique that employs antibodies to detect and quantify the amount of the protein. Results from different automated testing platforms may not be directly interchangeable, a factor that healthcare facilities account for.
The interpretation of S100B levels depends entirely on the clinical context. For example, a high level in a patient after a car accident would be interpreted in the context of a potential head injury. The same level in a patient with a known history of melanoma would instead suggest disease progression.
Factors outside of acute injury can also influence S100B levels, including extracranial injuries and vigorous physical activity. Healthcare professionals use the test as one piece of a larger diagnostic puzzle, combining the result with clinical evaluation and patient history. The test is not intended for self-diagnosis but serves as a tool to guide further medical decisions.