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External cause codes describe how an injury or event occurred and are added to the record to provide context about the incident, such as the mechanism, place of occurrence, and activity involved. They are used as secondary codes alongside the principal diagnosis to give a fuller picture for epidemiology, prevention, and appropriate billing, rather than changing which diagnosis is considered the main reason for the encounter. In practice, the principal diagnosis remains the injury or condition, and the external cause code adds the external context.

These codes are not used to replace the principal diagnosis, nor are they meant to indicate the clinician’s location or the provider’s specialty.