You were told the scan looked fine. You went home with a normal CT and a reassuring word. Weeks later the fog has not lifted, and somewhere in the back of your mind you have started to wonder whether you are imagining it.
You are not. A standard CT scan is very good at one job — finding blood and fractures that need a neurosurgeon tonight. It was never designed to find the kind of microscopic injury that produces persistent cognitive fog. A clear CT rules out an emergency. It does not rule out a concussion.
Watch on YouTube: Concussion: The Invisible Injury
Why the scan misses it
A concussion is not primarily a structural problem you can photograph. The damaging event happens at the scale of individual cells: axons — the long wires that carry signals between regions of the brain — are stretched and sheared by rapid acceleration and deceleration. The injury is real, and it is measured in microns. A CT scanner resolves millimetres.
This is why the question “is anything showing up on the picture?” is the wrong question. The better question is whether your brain has released the chemical evidence of injury into your bloodstream.
The proteins that leak when brain tissue is injured
When brain cells are damaged, their contents enter the blood. Two of these proteins are now measurable in routine clinical practice:
- GFAP (glial fibrillary acidic protein) comes from astrocytes — the support cells that scaffold and feed neurons.
- UCH-L1 (ubiquitin C-terminal hydrolase L1) comes from the neurons themselves.
Because they originate in different cell populations, the pair carries different information: broadly, GFAP speaks to astroglial injury and UCH-L1 to neuronal injury. Both rise within hours of trauma, which is what makes them useful early, when a patient is still deciding whether this is serious.
This part is established, not experimental. A combined GFAP/UCH-L1 blood test has been cleared by the FDA to help clinicians decide which head-injured patients actually need a CT scan. Used properly, it can spare people a radiation-heavy scan they were never going to benefit from.
The markers used when symptoms will not resolve
If symptoms persist beyond the acute window, two further proteins become relevant:
- Neurofilament light chain (NfL) is a structural protein from the axon itself. It can remain elevated for months, which makes it less a snapshot than a running account of ongoing axonal degeneration.
- Tau is associated with repetitive head impacts and is studied as a signal of longer-term cognitive risk.
Classify these honestly: NfL and tau are investigational in this setting. They are powerful research tools and are increasingly used clinically, but they are not the settled, cleared diagnostic that the GFAP/UCH-L1 pair has become for the acute CT decision. Anyone presenting a tau result as a diagnosis of long-term brain disease is overselling what the test can currently support.
The same caution applies to inflammatory cytokines and microRNA signatures, which we monitor as part of a broader picture of how a given nervous system is responding — not as standalone verdicts.
Looking past the blood
Two other lines of evidence matter when the standard imaging is unrevealing.
Advanced MRI. Diffusion tensor imaging (DTI) is sensitive to the integrity of white-matter tracts and can reveal disruption that conventional sequences do not. It is not what a standard machine produces by default.
Eye movement. The control of gaze runs through the brainstem, so the eyes are an unusually direct window into it. Eye tracking can pick up subtle errors in saccades and smooth pursuit — errors far too small to notice in yourself. These digital biomarkers have the practical virtue of measuring performance rather than mood: they do not depend on how you feel on the morning of the test, or on whether anyone believes you.
What most clinics will not tell you
Biomarkers are not a magic bullet, and you should be suspicious of anyone who sells them that way.
Current biomarkers have modest effect sizes, and values from injured people can overlap with values from healthy ones. That overlap is real and it is the honest limitation of the whole field. Despite FDA clearances, many insurers still refuse to cover these tests — a coverage decision, not a scientific one, but one that shapes what gets ordered.
This is exactly why no single number should decide anything. One data point is a hypothesis. A pattern across blood, imaging, ocular performance and your actual clinical history is evidence.
Frequently asked questions
Can I have a concussion if my CT scan was normal?
Yes. A CT scan is designed to detect bleeding and skull fractures that require emergency intervention. Concussion is largely a microscopic and functional injury, so a normal CT is expected in most concussions and does not mean you were not injured.
What do GFAP and UCH-L1 actually measure?
They are proteins released into the blood when brain cells are damaged. GFAP comes from astrocytes, the brain’s support cells; UCH-L1 comes from neurons. Both rise within hours of injury, and a combined test has FDA clearance to help decide whether a CT scan is warranted.
Are NfL and tau tests diagnostic for long-term brain damage?
No. Neurofilament light and tau are informative but should be treated as investigational in this context. NfL can stay elevated for months and reflects ongoing axonal degeneration; tau is associated with repetitive injury and studied as a risk signal. Neither establishes a diagnosis on its own.
Why would my insurance not cover a test the FDA cleared?
FDA clearance and insurance coverage are separate decisions. Many payers still decline to cover concussion biomarker testing despite clearance. That is a reimbursement policy judgment, and it is worth asking about cost before testing.
Where is Injury Experts located?
Injury Experts is at 4477 Woodson Rd, Suite 202, St. Louis, MO 63134, next to Lambert International Airport, serving the St. Louis region across Missouri and Illinois. Call (314) 887-5866 or text (314) 886-5902.
Get your injury validated by science, not by an adjuster's spreadsheet
We turn subjective pain into objective, court-ready evidence — biomarkers, biomechanics and diagnostic blocks. Bring us your case before the insurer defines it for you.
Or call (314) 887-5866 · text (314) 886-5902
Key takeaways
- A normal CT rules out a surgical emergency, not a concussion.
- GFAP and UCH-L1 are established, FDA-cleared aids to the CT decision.
- NfL, tau, cytokines and microRNA signatures are informative but investigational — not diagnoses.
- Eye tracking and DTI measure performance and structure that standard imaging and self-report both miss.
- Biomarker values overlap between injured and healthy people, so no single result should decide your care or your case.
Medically reviewed by Gurpreet Singh Padda, MD, MBA, MHP — Board Certified in Anesthesiology, Pain Medicine, Interventional Pain Management, Addiction Medicine, and Obesity Medicine.
This article is for general education and is not medical advice. It does not create a physician–patient relationship. Do not start, stop, or change any medication or treatment without consulting your physician. If you have had a head injury and develop worsening headache, repeated vomiting, seizures, weakness, or increasing confusion, seek emergency care immediately.
