Adverse events (AEs) are untoward medical occurrences in participants receiving a study intervention, regardless of whether they’re related to the study. Serious adverse events (SAEs) are AEs that meet regulatory seriousness criteria, while unanticipated adverse device effects (UADEs) are serious, unexpected effects associated with investigational medical devices. These commonly misunderstood terms have distinct regulatory definitions and reporting requirements that directly affect participant safety, study oversight, and regulatory compliance.
By understanding these differences, investigators, sponsors, contract research organizations (CROs), and institutional review boards (IRBs) can determine what to report, who to notify, and when action is required.
What is a serious adverse event (SAE)?
An SAE is an AE that meets established regulatory seriousness criteria. Under Food and Drug Administration (FDA) regulations and International Council for Harmonisation Good Clinical Practice (ICH GCP) guidelines, an event is considered serious if it:
- Results in death.
- Is life-threatening.
- Requires inpatient hospitalization.
- Prolongs an existing hospitalization.
- Results in persistent or significant disability or incapacity.
- Causes a congenital anomaly or birth defect.
- Requires medical intervention to prevent one of these outcomes.
The final category is often called an important medical event.
Severity and seriousness are not the same. Severity describes the intensity of an event—mild, moderate, or severe. Seriousness is a regulatory classification based on patient outcomes and determines reporting requirements. A severe headache may not be serious, while a mild allergic reaction requiring hospitalization may be considered serious.
Common SAE examples include stroke, myocardial infarction (heart attack), hospitalization due to severe infection, sepsis, anaphylaxis requiring emergency treatment, death, suicide attempt, and life-threatening arrhythmia.
Investigators typically notify the sponsor immediately, often within 24 hours per protocol. Sponsors evaluate the event for expedited reporting to regulatory authorities and IRBs when required, and follow-up information should be submitted as additional clinical details become available. Prompt reporting protects current and future participants, allows sponsors to reassess study risk, and may lead to protocol amendments, revised informed consent documents, or temporary enrollment holds.
What is an unanticipated adverse device effect (UADE)?
Under 21 Code of Federal Regulations (CFR) Part 812, a UADE is a serious adverse effect on the health or safety of a participant caused by, or associated with, an investigational medical device that was not previously identified in nature, severity, or incidence in the investigational plan or application.
UADEs apply only to investigational medical device studies conducted under an investigational device exemption (IDE). Unlike general AEs, they represent unexpected serious risks and often require expedited regulatory reporting.
Examples include a device component fracturing and causing internal bleeding, an implant migrating and causing permanent nerve damage, a software malfunction delivering an incorrect therapy dose, an investigational catheter causing a life-threatening complication, or a battery causing burns not identified in preclinical testing.
Investigators promptly report the UADE to the sponsor and reviewing IRB. Sponsors evaluate the event and report it to the FDA and participating investigators as required. The event may trigger a risk assessment, protocol modifications, additional monitoring, informed consent revisions, or temporary suspension of enrollment. UADE reporting helps identify previously unknown device risks, supports participant safety, and enables sponsors and regulators to determine whether the investigational device can continue to be studied safely.
| Adverse event (AE) | Serious adverse event (SAE) | Unanticipated adverse device effect (UADE) | |
| Definition | Any untoward medical occurrence in a participant receiving a study intervention, regardless of whether it is related to the study. | An adverse event that results in death, is life-threatening, requires or prolongs hospitalization, results in significant disability/incapacity, causes a congenital anomaly/birth defect, or is another medically important event. | Any serious adverse effect on the health or safety of a participant caused by, or associated with, a medical device that was not previously identified in nature, severity, or incidence in the investigational plan or application. |
| Applies to | Drug, biologic, and medical device studies | Drug, biologic, and medical device studies | Investigational medical device studies only |
| Must be serious? | No | Yes | Yes |
| Must be unexpected? | No | Not necessarily | Yes |
| Related to study intervention? | May or may not be related | May or may not be related | Must be associated with the investigational device |
| Examples | Mild headache, nausea, injection-site soreness, fatigue | Stroke, myocardial infarction, hospitalization for severe infection, death | Device malfunction causing unexpected internal injury, previously unknown device-related complication |
| Primary purpose of reporting | Document participant safety and identify trends | Prompt evaluation of significant participant risk and regulatory compliance | Identify previously unknown risks associated with an investigational medical device |
| Who initially reports it? | Investigator or study site | Investigator or study site | Investigator to sponsor and reviewing IRB |
| Typical recipients | Sponsor (per protocol) | Sponsor, IRB, and regulatory authorities as required | Sponsor, IRB, FDA (for IDE studies), and other investigators as required |
| Reporting timeline | Per study protocol and sponsor requirements | Expedited reporting according to protocol and applicable regulations | Expedited reporting under FDA IDE regulations (21 CFR Part 812) |
| Applicable regulations | ICH E6(R3), protocol-specific requirements | ICH E6(R3), 21 CFR Part 312 (drug/biologic studies), protocol requirements | 21 CFR Part 812 (investigational device exemptions) |
| Key takeaway | Every SAE is an AE, but not every AE is an SAE. | A subset of AEs that meet FDA seriousness criteria. | A subset of serious device-related events that are both unexpected and associated with an investigational device. |
How do AE, SAE, and UADE reporting requirements differ?
Investigators and study sites typically identify and initially report safety events to the sponsor, while sponsors evaluate those events and submit reports to IRBs, the FDA, and other regulatory recipients when required.
Applicable requirements include ICH E6(R3), 21 CFR Part 312 for drug and biologic studies, and 21 CFR Part 812 for investigational medical device studies.
Reporting timelines vary by event type and applicable requirements. AEs are generally reported according to the protocol and sponsor requirements, while SAEs and UADEs may require expedited reporting. Documentation should capture the event description, seriousness criteria, relationship assessment, actions taken, and relevant follow-up information.
| Event type | Report recipient(s) | General timeline | Applicable framework | Example scenario |
| AE | Sponsor, according to the protocol | Per protocol and sponsor requirements | ICH E6(R3) and protocol-specific requirements | A participant reports mild nausea after receiving the study intervention. |
| SAE | Sponsor and, when required, the IRB and regulatory authorities | Expedited reporting according to the protocol and applicable regulations | ICH E6(R3), 21 CFR Part 312, and protocol requirements | A participant is hospitalized for a severe infection. |
| UADE | Sponsor, reviewing IRB, FDA, and participating investigators as required | Expedited reporting under IDE requirements | 21 CFR Part 812 | An investigational implant unexpectedly migrates and causes permanent injury. |
Drug and biologic studies generally follow ICH E6(R3), 21 CFR Part 312, and protocol-specific requirements. Medical device studies must also follow 21 CFR Part 812.
Across multi-site studies, sponsors should standardize definitions, reporting forms, escalation pathways, and timelines. Sites should also confirm local IRB requirements rather than assume every IRB follows the same process.
Who is responsible for reporting safety events?
Investigators and sites identify, assess, document, and report safety events while ensuring participants receive appropriate care. Sponsors review safety data, determine reporting obligations, and communicate risks.
CROs may support safety-reporting operations, but sponsors retain oversight responsibility. IRBs review reportable safety information and may require changes to the protocol, informed consent process, monitoring plan, or study conduct.
The workflow moves from event identification through assessment, sponsor review, required reporting, follow-up, and protective action.
Best practices for accurate safety event reporting
These practices can help research teams improve reporting accuracy, consistency, and compliance across studies:
- Train study staff on AE, SAE, and UADE definitions.
- Use standardized documentation.
- Review protocol-specific reporting requirements.
- Perform periodic quality reviews for missing, inconsistent, or late reports.
Frequently asked questions
Clear reporting supports safer clinical research
Every SAE is an AE, but not every AE is serious, and UADEs are limited to serious, unexpected effects associated with investigational medical devices. Because reporting obligations vary by event type, protocol, study type, and applicable regulation, research teams must classify events accurately and route them to the appropriate recipients within required timelines.
Timely reporting enables sponsors, investigators, and IRBs to evaluate emerging risks, determine whether study changes are needed, and make informed decisions about ongoing study conduct. This helps protect current and future participants while supporting regulatory compliance.
