How Can a Concussion Be Identified or Diagnosed?

A concussion is diagnosed clinically, meaning no single scan, blood draw, or gadget can confirm one on its own. Instead, clinicians piece together a picture from what you report feeling, how your eyes and balance behave, how well you think and remember, and whether those findings line up with a plausible mechanism of injury. The process is genuinely multi-layered, and the tools used at the sideline minutes after a hit look very different from the ones used in a clinic days later. Understanding each layer helps explain why concussions still get missed and why researchers keep hunting for a definitive test.

Why There Is No Single Concussion Test

The core problem is that a concussion is a functional disruption of the brain, not a structural one. After a blow or jolt, ionic shifts and a burst of the neurotransmitter glutamate create a temporary energy crisis in the brain, demanding more fuel than injured cells can supply.

That metabolic mismatch produces the symptoms we associate with concussion, but it doesn’t leave the kind of visible damage a CT scan or standard MRI can detect. Conventional neuroimaging is typically normal in concussed patients.² Those scans are still ordered in many emergency departments, but their job is to rule out something worse, like a skull fracture or brain bleed, not to confirm a concussion.

Because the injury is metabolic and microscopic rather than structural, diagnosis depends on catching its downstream effects across several body systems: cognition, balance, eye movements, mood, and physical symptoms like headache and light sensitivity. No single domain tells the whole story. Someone can have a screaming headache but ace a memory test, or feel fine at rest but wobble the moment they stand on one foot with their eyes shut. That’s why modern concussion assessment is explicitly multimodal.

Sideline Assessment and Symptom Reporting

The first chance to catch a concussion often comes within minutes of the injury, on a sideline or at the scene. Here, symptom checklists remain the most sensitive single tool available. A prospective study of college athletes compared several established sideline tests head-to-head and found that self-reported symptom scores had the best combination of sensitivity and specificity for predicting a clinical concussion diagnosis. The symptom severity score, in particular, correctly flagged roughly nine out of ten concussed athletes when it rose by at least three points from baseline.³

The standardized tool used most widely in sports is the Sport Concussion Assessment Tool, now in its sixth edition (SCAT6). It bundles a symptom checklist with a brief cognitive screen (the Standardized Assessment of Concussion, or SAC), a balance component, and a neck exam. The SAC, taken alone, fared poorly in that same head-to-head comparison, picking up fewer than half of concussed athletes.³ That’s the whole rationale for bundling tests: what one misses, another catches.

There’s a catch with symptom reporting, though. It relies on honest self-assessment, and athletes sometimes downplay symptoms to stay in a game. Also, symptoms like headache, dizziness, and fatigue are common in everyday life and after vigorous exertion, so a checklist alone can’t distinguish a concussion from a rough day. Having a baseline score taken before the season starts makes post-injury changes much easier to spot.

Eye Movements and Vestibular Screening

One of the more revealing exam components involves watching how a person’s eyes track objects and how they respond to head movement. The Vestibular/Ocular Motor Screening (VOMS) tests smooth pursuits, saccades (quick gaze shifts), the vestibulo-ocular reflex (keeping your vision stable while your head turns), visual motion sensitivity, and near-point convergence (the closest distance at which you can focus on an approaching object without seeing double).

The VOMS has shown strong ability to identify concussed patients. In an early validation study, the vestibulo-ocular reflex and visual motion sensitivity components were the most predictive of concussion, and a statistical model incorporating several VOMS items produced high accuracy for distinguishing concussed from non-concussed patients.⁴ A near-point convergence distance of five centimeters or more, or any VOMS item symptom score of two or higher, substantially increased the probability of correctly identifying a concussed person.

More recent work from the NCAA-DoD CARE consortium confirmed that the VOMS adds genuine value when used alongside the SCAT. Used alone, the SCAT3 achieved an area-under-the-curve of about 0.79 for acute concussion identification; adding the VOMS pushed that to roughly 0.84.⁵ A systematic review covering non-severe head injuries more broadly found VOMS sensitivity ranging from about 58% to 96% and specificity from 46% to 92%, depending on the population and cutoff used.⁶ The ranges are wide, but the screening consistently adds something that symptom checklists miss.

Another eye-tracking tool worth mentioning is the King-Devick test, which measures how fast someone can read numbers off cards. It performed well in the sideline comparison study, with an 85% sensitivity rate for any increase in completion time relative to baseline.³ Its appeal is speed: it takes under two minutes.

Balance Testing

Postural stability takes a hit after concussion, and clinicians have long used this to their advantage. The Balance Error Scoring System (BESS) asks a person to hold three stances (double-leg, single-leg, tandem) on firm ground and then on foam, eyes closed. An observer counts balance errors over 20-second intervals. Concussed athletes averaged about 17 errors the day after injury, compared with roughly 8 in healthy controls, but those deficits returned to baseline within three to five days.⁷

The BESS is cheap and needs no equipment beyond a foam pad, which is why it’s been embedded in sideline assessment batteries for years. It reliably distinguishes concussed young adults from healthy ones in the acute phase.⁸ But it has a weakness: human scoring is subjective, and subtle sway that a trained eye might miss can matter. Research using wearable inertial sensors during the same BESS stances found that the sensors objectively detected group-level differences in acutely concussed athletes that the standard clinical BESS scoring did not pick up.⁹ The simple act of standing on both feet with eyes open, which is the easiest condition, was actually where sensor-measured side-to-side sway best classified concussed athletes. It’s a reminder that technology can squeeze more signal out of familiar clinical tests.

Computerized Neurocognitive Testing

Programs like ImPACT are used extensively in sports to track reaction time, processing speed, and memory before and after a suspected concussion. The logic is straightforward: if your post-injury scores drop meaningfully from your own baseline, something is off. These tools have value, but they come with important caveats.

Test-retest reliability is only moderate. When healthy people took ImPACT and two similar programs twice, the consistency of their scores ranged from low to moderate across testing intervals, meaning a person can score quite differently on two occasions even without any injury.¹⁰ That means small score changes after a hit may reflect normal test variability rather than a brain injury.

When individual baselines aren’t available, clinicians compare to normative data, but those norms aren’t one-size-fits-all. Urban high school athletes, for instance, scored below the age-dependent normative values built into ImPACT, suggesting that socioeconomic factors influence baseline performance.¹¹ Using generic norms for these athletes could make a post-injury score look normal when it actually represents a decline. This is why individual baselines are strongly preferred, and why neurocognitive test results should never be the sole basis for a return-to-play decision.

Interestingly, a meta-analysis found that having one or even multiple prior concussions had little meaningful effect on baseline ImPACT scores, aside from a small decrease in visual memory after a single prior concussion.¹² So baseline testing doesn’t quietly drift downward with concussion history the way some people assume.

Blood Biomarkers

The idea of a simple blood test for concussion has gotten closer to reality, though we’re not fully there yet. The protein GFAP (glial fibrillary acidic protein) has emerged as the leading candidate. In a large study, GFAP drawn within 30 minutes of a traumatic brain injury achieved an area-under-the-curve of 0.88 for detecting CT-visible brain lesions, and at a particular threshold, its sensitivity hit about 98%.¹³ The FDA has already cleared a blood test combining GFAP and UCH-L1 to help determine whether an adult with a suspected mild traumatic brain injury needs a CT scan. A meta-analysis confirmed that GFAP outperforms UCH-L1 overall, with better specificity.¹⁴

But here’s the nuance: these blood biomarkers are best at ruling out structural damage visible on imaging, which is a slightly different question from confirming a concussion. Many concussions produce no CT-visible lesion at all. A negative GFAP result can give you confidence that a CT is unnecessary, but it doesn’t mean the person isn’t concussed. Researchers are working on connecting biomarker levels to clinical outcomes beyond imaging, and early data suggest very high GFAP levels are associated with worse clinical trajectories. Still, a blood test alone won’t replace the multi-domain clinical assessment anytime soon.

Saliva-Based Biomarkers on the Horizon

Blood draws aren’t always practical on a sideline, so researchers have been exploring saliva as an alternative. Small RNA molecules called microRNAs, which help regulate gene expression, show altered patterns after head injuries. In a study of male rugby players, a panel of 14 small non-coding RNAs measured in saliva could distinguish concussed players from uninjured ones with high accuracy both immediately after the game and 36 to 48 hours later. When tested prospectively, the panel’s area-under-the-curve reached 0.96 post-game.¹⁵

A challenge with saliva microRNAs is that exercise itself and cumulative contact-sport participation can shift the same molecules. Researchers have tried to filter out those confounders. One study identified a single microRNA ratio that achieved about 82% sensitivity and 73% specificity for differentiating concussed from non-concussed athletes, after excluding microRNAs affected by exercise and season-long contact exposure.¹⁶ Separate research has also found that salivary microRNAs differ in people with a history of multiple concussions, hinting at a possible tool for tracking cumulative injury burden.¹⁷ None of this is ready for routine clinical use, but the pipeline is real and well-funded.

Advanced Imaging and Brain Activity Measures

While standard CT and MRI miss concussions, more specialized imaging techniques can pick up microstructural changes. Diffusion tensor imaging (DTI), a type of MRI that maps the movement of water through nerve fibers, has been used in hundreds of studies to detect disrupted white-matter tracts after concussion.¹⁸ One study found that DTI measures in frontal and temporal brain regions correlated with cognitive screening scores in concussed patients.¹⁹ The research is promising, but DTI remains a research tool rather than a standard diagnostic one; it requires specialized scanner protocols, careful analysis, and isn’t available in a typical emergency setting.

Electrophysiological approaches offer another angle. Quantitative EEG (qEEG) and event-related potentials (ERPs), which measure brain electrical responses to specific stimuli, have shown some promise. Individual qEEG measures have limited diagnostic power on their own, but when combined into statistical models, they show significant utility for detecting mild traumatic brain injury. ERPs appear to be particularly useful, picking up abnormalities even in cases where standard EEG looks normal.²⁰ One machine-learning study using ERP features and wavelet analysis achieved about 81% classification accuracy for detecting concussion effects in retired athletes, some of whom had sustained their last injury decades earlier.²¹

Diagnosing Concussion in Children

Children aren’t small adults when it comes to concussion diagnosis. Their brains are still developing, their ability to describe symptoms varies enormously by age, and their baseline behavior can look different from week to week even without an injury. The latest consensus tools reflect this. The Child SCAT6 is designed for children aged eight to twelve, while the SCAT6 covers those over thirteen. For younger children aged five to nine, the VOMS-Child (VOMS-C) uses simpler yes-or-no symptom reporting that’s developmentally appropriate.²²

Research on the earlier Child SCAT-5 found it useful for tracking common symptoms and identifying improvements in neurocognitive functioning, especially memory, across clinic visits in children aged five to nine.²³ For children, clinicians often rely more heavily on parent and teacher observations than on self-reporting, particularly for mood changes, sleep disruption, or behavioral shifts that a young child may not articulate.

Pupillometry, which measures how the pupils constrict and dilate in response to light, has been explored as an objective concussion marker in children. In one study, specific pupil recovery-time measures differed between concussed and non-concussed children, though the differences were modest and varied by age group.²⁴ It’s an interesting signal, but not one that’s ready to anchor a diagnosis.

Sex Differences in Symptom Presentation

How a concussion shows up can differ between males and females, which matters for diagnosis because many tools rely on what symptoms the person reports. In high school athletes, males more frequently reported amnesia and confusion after concussion, while females more frequently reported drowsiness and noise sensitivity.²⁵ The total number of symptoms didn’t differ, and neither did recovery timelines, but the types of symptoms diverged in a pattern that’s appeared across multiple studies.

A systematic review and meta-analysis confirmed that at baseline, before any injury, females reported higher scores on standard symptom scales, endorsing more headache, difficulty concentrating, energy complaints, and emotional symptoms than males. After concussion, the only statistically significant symptom difference was that females were less likely to report confusion. The overall symptom score difference, while statistically present at baseline, was clinically insignificant when back-transformed to real-world magnitudes.²⁶ The practical takeaway is that sex-specific baseline norms matter. A female athlete’s higher baseline symptom score isn’t a red flag; it’s normal. If you compare her post-injury scores to male norms, you could overestimate the change and her injury severity, or underestimate it if her usual baseline was already elevated.

The Cervical Spine Complication

Any force strong enough to rattle the brain also jars the neck, and cervical spine injuries can mimic concussion symptoms almost perfectly: headache, dizziness, difficulty concentrating, and visual disturbances all overlap. This makes the neck a critical part of the differential diagnosis. Clinical tests such as the cervical joint-reposition error test, the smooth-pursuit neck-torsion test, and physical examination of the cervical spine can help distinguish neck-driven symptoms from brain-driven ones.²⁷ Patient-reported measures like the Neck Disability Index and Dizziness Handicap Inventory can also flag cervicogenic symptoms that need further workup.²⁸

This matters clinically because the treatment paths diverge. A neck injury may respond well to manual therapy and targeted exercises, while a concussion requires a different management strategy involving graduated cognitive and physical activity. If both are present, which is common, missing the cervical component can stall recovery and lead to months of persistent symptoms that get blamed entirely on the brain.

Effort Testing and Malingering

Concussion diagnosis is unusual in medicine because it leans so heavily on what the patient reports. That creates vulnerability in both directions: underreporting in an athlete who wants to stay on the field, and overreporting in someone with an incentive to appear more impaired than they are (legal claims, prolonged time off work, academic accommodations). Symptom validity tests (SVTs) are structured, objectively scored instruments psychologists use to detect poor effort, underperformance, and exaggeration.²⁹

These tests work on a clever principle: many of them are designed to be easy enough that even a person with a genuine brain injury should pass. Performance well above a designated cut score suggests valid effort; performance near or below chance, performing worse than someone guessing randomly, is the strongest neuropsychological indicator of deliberate faking.³⁰ Effort testing is now considered a standard part of formal neuropsychological evaluation after concussion, and failing it doesn’t necessarily mean someone is lying. Factors like extreme fatigue, pain, or psychological distress can also depress effort. But it does signal that the other test results need to be interpreted cautiously.

When “Recovered” Isn’t Fully Healed

One of the more unsettling findings in recent concussion science is that the brain may still be dealing with injury after clinical tests say a person has recovered. A systematic review found that multiple studies show physiological dysfunction outlasting current clinical measures of recovery, which has been used to support the idea of a buffer zone, a period of gradually increasing activity before returning to full contact.³¹

A striking illustration came from a PET imaging study of collegiate athletes. After sustaining a concussion, athletes completed standard clinical measures (ImPACT, SCAT5, balance testing) that showed they had returned to normal. Yet PET scans revealed persistently elevated brain inflammation, consistent with microglial activation, in limbic system regions, the dorsal striatum, and the medial temporal lobe.³² This finding is part of why concussion experts advocate for conservative return-to-play timelines and why suffering a second concussion before the first has fully resolved is considered especially dangerous. The clinical tools we have are good at tracking the symptoms a person notices, but they can’t yet see everything happening beneath the surface.

1
PubMed Central. The new neurometabolic cascade of concussion
2
PubMed Central. Imaging of Concussion in Young Athletes
3
PubMed. Diagnostic accuracy and reliability of sideline concussion evaluation: a prospective, case-controlled study in college athletes comparing newer tools and established tests
4
PubMed Central. A Brief Vestibular/Ocular Motor Screening (VOMS) Assessment to Evaluate Concussions: Preliminary Findings
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PubMed. Optimizing VOMS for identifying acute concussion in collegiate athletes: Findings from the NCAA-DoD CARE consortium
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Journal of Concussion. Vestibular/ocular motor screening (VOMS) score for identification of concussion in cases of non-severe head injury: A systematic review
7
PubMed Central. Systematic Review of the Balance Error Scoring System
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PubMed Central. Comparison of the balance accelerometer measure and balance error scoring system in adolescent concussions in sports
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PubMed Central. Sensor-Based Balance Measures Outperform Modified Balance Error Scoring System in Identifying Acute Concussion
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PubMed Central. Test-retest reliability of computerized concussion assessment programs
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PubMed Central. Comparison of Urban versus Industry Normative Values of Immediate Post-Concussion Assessment and Cognitive Testing (ImPACT™)
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PubMed Central. Cumulative Effects of Concussion History on Baseline Computerized Neurocognitive Test Scores: Systematic Review and Meta-analysis
13
JAMA Network Open. Diagnostic Performance of GFAP, UCH-L1, and MAP-2 Within 30 and 60 Minutes of Traumatic Brain Injury
14
PubMed. Systematic review and meta-analysis of observational studies evaluating glial fibrillary acidic protein (GFAP) and ubiquitin C-terminal hydrolase L1 (UCHL1) as blood biomarkers of mild acute traumatic brain injury (mTBI) or sport-related concussion (SRC) in adult subjects
15
British Journal of Sports Medicine. Unique diagnostic signatures of concussion in the saliva of male athletes: the Study of Concussion in Rugby Union through MicroRNAs (SCRUM)
16
PubMed Central. Refinement of saliva microRNA biomarkers for sports-related concussion
17
PubMed Central. Saliva microRNA Biomarkers of Cumulative Concussion
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PubMed Central. Role of Diffusion Tensor Imaging in the Diagnosis of Traumatic Axonal Injury in Individual Patients with a Concussion or Mild Traumatic Brain Injury: A Mini-Review
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PubMed Central. Diffusion Tensor Imaging Correlates of Concussion Related Cognitive Impairment
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PubMed Central. Traumatic brain injury detection using electrophysiological methods
21
PubMed. From Group-Level Statistics to Single-Subject Prediction: Machine Learning Detection of Concussion in Retired Athletes
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PubMed. Age-appropriate assessment of concussion in children and adolescents
23
Archives of Clinical Neuropsychology. A-18 Utility of the Child SCAT-5: Performance Differences Across Assessments in Pediatric Concussion
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PubMed. Evaluation of Quantitative Pupillometry in Acute Postinjury Pediatric Concussion
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PubMed Central. Sex differences in concussion symptoms of high school athletes
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PubMed. Differences in Symptom Reporting Between Males and Females at Baseline and After a Sports-Related Concussion: A Systematic Review and Meta-Analysis
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PubMed Central. Cervical Injury Assessments for Concussion Evaluation: A Review
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PubMed. The Role of Cervical Symptoms in Post-concussion Management: A Systematic Review
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PubMed Central. Symptom exaggeration and symptom validity testing in persons with medically unexplained neurologic presentations
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PubMed Central. Effort, symptom validity testing, performance validity testing and traumatic brain injury
31
British Journal of Sports Medicine. What is the physiological time to recovery after concussion? A systematic review
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PubMed Central. Microglial activation persists beyond clinical recovery following sport concussion in collegiate athletes