The NEXUS Head CT instrument is a clinical decision instrument to identify blunt head injury patients at very low risk of clinically important intracranial injury, in whom non-contrast head CT can usually be omitted.

Derivation and validation

The rule was derived and validated within the National Emergency X-Radiography Utilization Study (NEXUS II) programme as large prospective cohorts of blunt head injury patients undergoing head CT in North American emergency departments.

Study	Patients and setting	Primary outcomes	Key performance
Derivation (J Trauma 2005)	13,728 blunt head injury patients undergoing head CT in 21 North American emergency departments.	Any intracranial injury on CT.	Low-risk group about 13% of the cohort. Sensitivity about 98% for intracranial injury.
Validation (PLOS Med 2017)	11,770 consecutive blunt head injury patients undergoing head CT in four US emergency departments.	Neurosurgical intervention within 7 days. Clinically significant intracranial injury on CT.	Sensitivity 100% for neurosurgical intervention and 99% for clinically significant injury. Low-risk group about one-quarter of imaged patients with negative predictive value around 99.7% for significant injury.

Implementation notes

NEXUS Head CT was designed for broad use in adult blunt head injury presentations, excluding penetrating trauma, delayed presentations (>24 hours after injury) and non-traumatic indications for imaging.
In the validation study, criteria that could not be assessed and patients judged clinically unstable were treated as abnormal and not assigned low-risk status.
This calculator is implemented for adults. Use paediatric-specific head injury rules and local pathways in children.
Australian Choosing Wisely and radiology guidance support using validated head CT decision rules and avoiding CT in low-risk head injury patients when decision rule criteria are not met.

NEXUS Head CT Instrument for Blunt Head Injury