The Canadian CT Head Rule provides a binary recommendation: CT is indicated or CT is not required. If any of the four high-risk criteria are positive (GCS below 15 at 2 hours post-injury, suspected open or depressed skull fracture, two or more episodes of vomiting, or age 65 or older), a CT scan is recommended to evaluate for findings that may require neurosurgical intervention. The high-risk criteria have 100% sensitivity for neurosurgical lesions in the original validation study.

If no high-risk criteria are present but one or both medium-risk criteria are positive (retrograde amnesia exceeding 30 minutes, dangerous mechanism of injury), CT is also recommended to detect brain injuries visible on imaging even if they do not require surgery. If all criteria are negative, the probability of a clinically important brain injury is extremely low, and CT can be safely deferred, reducing unnecessary radiation exposure and healthcare costs.

Use the Canadian CT Head Rule for adult patients (age 16 and older) who present to the emergency department with a minor head injury, defined as witnessed loss of consciousness, definite amnesia, or witnessed disorientation in a patient with a Glasgow Coma Scale score of 13-15. The rule is designed to reduce the rate of unnecessary CT scans in this large patient population while maintaining near-perfect sensitivity for clinically significant injuries.

The rule should NOT be applied to patients with penetrating injuries, patients on anticoagulant therapy or with bleeding disorders, patients with obvious open skull fractures, patients who had a seizure after the injury, or patients with GCS below 13. These patients require CT regardless of the rule's criteria. The rule is also not validated for children under 16, for whom separate pediatric decision rules (PECARN, CATCH, CHALICE) should be used.

The Canadian CT Head Rule was derived and validated in Canadian emergency departments and may have different performance characteristics in other settings. Some validation studies have shown lower specificity than the original study, meaning the rule may still recommend CT for a substantial proportion of patients who do not have significant findings.

The definition of 'dangerous mechanism' is somewhat subjective (pedestrian struck by motor vehicle, occupant ejected from vehicle, fall from height greater than 3 feet or 5 stairs), and clinical judgment is needed in borderline cases. The rule also does not account for patients on antiplatelet agents (only anticoagulants are excluded), despite growing evidence that antiplatelet therapy may increase intracranial bleeding risk. Many clinicians have a lower threshold for imaging in elderly patients on antiplatelets. Finally, the rule addresses the need for imaging at a single time point and does not eliminate the need for clinical re-evaluation if symptoms worsen.