What does “Adverse event” mean?
Short answer
An adverse event is patient harm caused by care, not by the illness. Groups define it a bit differently, so say which one you use.
Also known as: AE, adverse patient event
How groups define it
| Group | Its definition |
|---|---|
| CMS, nursing homes (42 CFR 483.5) | A bad, usually unexpected event that causes death or serious injury, or the risk of it. |
| HHS Office of Inspector General | Harm to a patient from medical care. Its 2010 hospital study counted short-term harm apart. |
| Harvard Medical Practice Study (cited by AHRQ PSNet) | An injury from medical care that lengthened the stay or caused disability at discharge. |
| Joint Commission | Sentinel events are the most serious kind. |
In the rules
CMS wants adverse events tracked inside QAPI, its quality program. Hospitals, surgery centers, hospices and home health agencies must track them (42 CFR 482.21, 416.43, 418.58 and 484.65). Nursing homes must find, report, track, investigate and study them (42 CFR 483.75(c)(4)).
Example: a resident gets a double dose of a blood thinner and goes to the hospital. The facility logs it, finds causes, assigns fixes and tells its quality committee.
Mix-up: an adverse event is not always a mistake. A known drug reaction is one. A mistake caught in time is a near miss.
Frequently asked questions
Is an adverse event the same as a medical error?
No. An error is a mistake. An adverse event is harm. Some adverse events involve no mistake, and many mistakes cause no harm.
Which adverse events must be reported outside the organization?
It depends on the event, setting and state. CMS requires tracking inside QAPI. State laws and device rules may require outside reports.
What is the difference between an adverse event and a sentinel event?
A sentinel event is the most serious kind. It reaches the patient and causes death, severe harm or permanent harm. It needs a full analysis.
Sources
- AHRQ PSNet: Adverse events, near misses, and errors (primer)
- 42 CFR 483.5: Definitions (long-term care facilities)
- HHS OIG: Adverse Events in Hospitals: National Incidence Among Medicare Beneficiaries (OEI-06-09-00090, November 2010)
- Joint Commission: Sentinel Event Policy (SE chapter), Comprehensive Accreditation Manual, Update 1, July 2026
- 42 CFR 482.21: Hospital QAPI condition of participation
- 42 CFR 416.43: ASC QAPI condition for coverage
- 42 CFR 483.75: Nursing home QAPI
Reviewed against the sources above on Oct 5, 2026. Rules change: confirm current requirements with the issuing body or your counsel before relying on any summary.
Put the definition to work.
IncidentKit turns these terms into workflow: reports, investigations, corrective actions and packets.