Unobserved Is Not Absent: Prehospital Response Intervals and Diagnostic Visibility in Mild Traumatic Brain Injury

Mild traumatic brain injury diagnosis depends on observing transient neurological signs-loss of consciousness, altered mental status, and post-traumatic amnesia-that resolve within minutes. When clinical contact is delayed, these defining features may no longer be present for evaluation.

We analyzed 167 082 head injury encounters from the 2024 National Emergency Medical Services Information System dataset. Using k-means clustering, we identified 3 response profiles based on call-to-patient intervals. National Emergency Medical Services Information System was used as system-level evidence regarding prehospital response interval distributions rather than for individual diagnostic adjudication. Diagnostic visibility was modeled as the probability that defining signs remained observable at first clinical contact, integrating empirically derived sign duration estimates with prehospital response interval distributions.

Combined diagnostic visibility declined from 0.93 in Near-Scene encounters (median 6.9 minutes) to 0.44 in Remote-Response encounters (median 19.3 minutes). Rural and frontier populations were disproportionately represented in the Remote-Response cluster, where modeling estimates indicate that in 56% of Remote-Response encounters, the probability of any observable defining sign at first clinical contact was zero. Convergent findings from the French OSCOUR emergency department database suggest that these timing constraints operate across healthcare systems.

Diagnostic uncertainty in mild traumatic brain injury frequently reflects timing constraints rather than clinical failure. Aligning diagnostic frameworks with this reality supports structured retrospective reconstruction and addresses geographic inequities in diagnostic validity. Recognition of this constraint has direct implications for clinical guidelines, epidemiologic inference, and medicolegal interpretation.