Published by Unseen Progress, an independent publisher of caregiver research. Last reviewed 2026-07-10. Part of the TBI caregiver research overview.
Short answer. TBI severity — mild, moderate, or severe — is classified using three acute-phase measures: the Glasgow Coma Scale score (Teasdale & Jennett, 1974), the duration of loss of consciousness, and the length of post-traumatic amnesia (how long the survivor could not form continuous new memories after the injury). Roughly, a Glasgow Coma Scale of 13–15 is mild, 9–12 is moderate, and 3–8 is severe. Severity is a strong statistical predictor of the shape of recovery — higher severity generally means a longer, deeper recovery arc (Dikmen et al., 2009) — but it is a population-level probability, not a verdict on any one survivor's ceiling. The CDC and the Brain Injury Association of America both frame severity as the starting point of the story, not the ending.
Severity is assigned in the acute phase, usually in the emergency department, from three things.
1. Glasgow Coma Scale (GCS). Introduced by Graham Teasdale and Bryan Jennett at Glasgow in 1974 (Teasdale & Jennett, 1974) to standardize how clinicians describe a patient's level of consciousness, the GCS scores three responses — eye opening, verbal response, and motor response — and sums them into a number from 3 (deep unconsciousness) to 15 (fully alert). It was designed precisely to remove the ambiguity from phrases like "semi-conscious" and give teams a shared, comparable number. It remains one of the most widely used measures in all of clinical neurology.
2. Loss of consciousness (LOC). How long the survivor was unconscious. Seconds-to-minutes points toward mild; hours or more points toward severe.
3. Post-traumatic amnesia (PTA). The period after the injury during which the survivor could not lay down continuous new memories — often the most predictive of the three for long-term outcome. A survivor may be awake and talking during PTA but retains no ongoing memory of it. Longer PTA correlates with a longer recovery arc.
These three do not always agree, and clinicians weigh them together rather than mechanically. A survivor can have a reassuring early GCS but a long PTA, and the PTA is often the truer signal.
| Mild TBI | Moderate TBI | Severe TBI | |
|---|---|---|---|
| Glasgow Coma Scale | 13–15 | 9–12 | 3–8 |
| Loss of consciousness | 0–30 minutes | 30 min – ~24 hours | More than ~24 hours |
| Post-traumatic amnesia | Under ~24 hours | ~1–7 days | More than ~7 days |
| Typical family experience | "Concussion"; symptoms often invisible; frequently under-recognized | Clear, lasting cognitive and behavioral change; long rehabilitation | Prolonged acute/ICU phase; profound early impairment; multi-year recovery arc |
| What severity predicts | Usually a shorter arc, but persistent symptoms still possible | A longer, multi-domain recovery | The longest, deepest arc — but still a recovering trajectory |
The bands above are the widely used clinical ranges (CDC; Brain Injury Association of America). Exact cut-offs vary slightly between institutions, which is why a single borderline number is never the whole picture.
A note families deserve: "mild" is a clinical label about the acute injury, not a promise about daily life. A mild TBI can still leave real, lasting symptoms — this is exactly the "they look fine but aren't" pattern. The word describes the initial injury depth, not how hard the aftermath feels. See they look fine to outsiders.
Severity is one of the strongest predictors in the TBI outcome literature — but it predicts distributions, not individuals. Dikmen and colleagues (Dikmen et al., 2009), following cognitive outcomes after TBI, found that injury severity shaped the depth and duration of cognitive deficits: more severe injuries produced larger initial deficits and longer recovery. That is a statement about averages across many survivors.
What it is not is a fixed ceiling for a particular person. Two survivors with the same GCS can land in very different places years later, because outcome also depends on age, the specific regions injured, rehabilitation access and intensity, sleep, medical complications, and social support. The severity number sets the starting conditions of the recovery curve; it does not draw the endpoint.
This distinction matters enormously for families, because severity is often delivered as a sentence rather than a starting point. A family told "severe TBI" in week one can hear "this is permanent," when the literature says the opposite: even severe injuries follow a recovering trajectory that continues for years. See is TBI recovery permanent?.
It does help with:
It does not tell you:
The most useful stance is to treat severity as the weather forecast at the start of a long journey: real information about conditions, useless as a prediction of the exact destination.
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