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Short answer
GFAP (glial fibrillary acidic protein) is the main intermediate filament protein of mature astrocytes, which is why it is the standard astrocyte marker in neuroscience research. It is also measured in blood as a brain-injury biomarker, a separate use with its own regulatory status.
This page is general reference for researchers and laboratory staff. It is not medical advice and does not describe how to diagnose or treat any condition.
Definition and scope
GFAP is a cytoskeletal protein. In a 2011 review in Progress in Neurobiology, Middeldorp and Hol describe it as the main intermediate filament protein in mature astrocytes and an important cytoskeletal component of astrocytes during development. The same review notes that GFAP has several splice variants (isoforms) and that it has been linked to regeneration, synaptic plasticity and reactive gliosis.
Two different meanings of “GFAP marker” are in circulation, and they should not be mixed up:
- Cell-identity marker. GFAP signal in tissue or culture is used to identify astrocytes.
- Circulating injury biomarker. GFAP concentration in blood is used as a measure of glial injury after head trauma.
How GFAP is used in practice
GFAP as an astrocyte marker (gfap marker)
Because GFAP is abundant in mature astrocytes, labelling for it is a common way to flag astrocytes and to look for changes associated with reactive gliosis. Middeldorp and Hol also report that astrocyte subpopulations differ in which intermediate filaments and which GFAP isoforms they express. In practice that means a GFAP-positive result is not a statement about one uniform cell type or one isoform: the result depends on what the reagent detects and on the astrocyte population being examined.
For related bench context, see CASRAI’s guides on Cre-lox conditional knockout driver lines (where astrocyte-targeted drivers are used), two-photon microscopy and the Golgi stain for other ways of visualising nervous-system cells.
GFAP as a blood biomarker
Okonkwo and colleagues (Journal of Neurotrauma 2020;37(23):2460-2467) describe GFAP and S100B as markers of glial injury. In their report, a serum ELISA measuring GFAP together with UCH-L1 is FDA-cleared to help determine the need for head CT within 12 hours after mild traumatic brain injury (Glasgow Coma Scale 13-15). They also describe a point-of-care prototype that measured plasma GFAP in under 15 minutes, and report an area under the curve of 0.85 for GFAP versus 0.67 for S100B for CT abnormalities. Their GFAP measurements captured GFAP and its breakdown products.
Types or related concepts: marker versus biomarker
| Question | Astrocyte marker (tissue or cells) | Blood injury biomarker |
|---|---|---|
| What is read out | GFAP as a cytoskeletal protein of mature (and developing) astrocytes | GFAP and breakdown products circulating after glial injury |
| Purpose | Identify astrocytes; examine reactive gliosis and related states | Help evaluate suspected mild TBI alongside other clinical information |
| Main caveat from the sources | Isoforms and astrocyte subpopulations differ, so “GFAP-positive” is not one uniform population | Cleared use is an aid in deciding on head imaging, not a stand-alone diagnosis |
| Source | Middeldorp and Hol 2011 | Okonkwo et al. 2020; FDA classification 866.5830 |
Where it matters for labs and clinics
The openFDA 510(k) database lists record DEN170045: the Banyan BTI (Brain Trauma Assessment Test) from Banyan Biomarkers, Inc., a De Novo clearance with decision date 14 February 2018, product code QAT, regulation number 866.5830, device class 2. The FDA classification text for 866.5830 describes the device as intended for in vitro diagnostic use as an aid in the evaluation of patients with suspected mild TBI, in conjunction with other clinical information, to assist in determining the need for radiologic (for example CT or MR) head imaging per current standard of care.
The practical consequences for laboratory staff are limited to what that record says. The cleared indication belongs to that specific in vitro diagnostic assay. It does not transfer to other GFAP reagents or kits, and it does not describe a research assay for tissue or culture work. For how regulators treat biomarkers more broadly, see the FDA Biomarker Qualification Program entry. For the immunoassay format commonly used to measure proteins in solution, see the sandwich ELISA guide.
Common misunderstandings
“GFAP-positive means astrocyte, full stop”
GFAP is the main intermediate filament protein of mature astrocytes, but astrocyte subpopulations differ in GFAP isoform expression, and the protein is also described in developing astrocytes and in reactive gliosis. Interpret a GFAP signal in light of the reagent and the tissue state.
“An elevated blood GFAP result diagnoses brain injury”
The FDA-classified use is an aid, in conjunction with other clinical information, in evaluating suspected mild TBI and in deciding whether head imaging is needed. It is not described as a diagnosis on its own.
“GFAP and S100B are interchangeable”
Both are called markers of glial injury, but in the Okonkwo et al. report they performed differently for CT abnormalities (AUC 0.85 for GFAP versus 0.67 for S100B).
Frequently asked questions
What does GFAP stand for?
Glial fibrillary acidic protein. It is the main intermediate filament protein in mature astrocytes (Middeldorp and Hol, 2011).
What is a GFAP marker?
Either GFAP used to identify astrocytes in research material, or GFAP measured in blood as a marker of glial injury. The two uses have different readouts and different regulatory status.
Is there an FDA-cleared GFAP test?
The openFDA record DEN170045 (Banyan BTI, De Novo, 2018, regulation 866.5830, class 2) is an in vitro diagnostic that includes GFAP measurement and is classified as an aid in evaluating suspected mild TBI. Read the record itself for its exact scope.
Sources
- Middeldorp J, Hol EM. GFAP in health and disease. Prog Neurobiol 2011;93(3):421-443. https://pubmed.ncbi.nlm.nih.gov/21219963/ (read 2026-11-08).
- Okonkwo DO et al. J Neurotrauma 2020;37(23):2460-2467. https://pmc.ncbi.nlm.nih.gov/articles/PMC7698990/ (read 2026-11-08).
- openFDA device 510(k) record DEN170045: https://api.fda.gov/device/510k.json?search=k_number:DEN170045 (read 2026-11-08).
- openFDA device classification, 21 CFR 866.5830, product code QAT: https://api.fda.gov/device/classification.json?search=product_code:QAT (read 2026-11-08).
