• Patient Safety Primer
  • Last Updated: July 2016

Adverse Events, Near Misses, and Errors


The concept of medical harm has existed since antiquity, famously discussed by Hippocrates and passed on in the word iatrogenesis, from the Greek for originating from a physician. The topic has received attention from notable physicians in the centuries since. A paper in the New England Journal of Medicine in 1956 discussed the topic of diseases of medical progress, and this paper evolved into a book that used the phrase "iatrogenic disease" in its title.

One of the first studies that sought to quantify the incidence of iatrogenic harm was the Medical Insurance Feasibility Study, funded by the California Medical Association and the California Hospital Association. This study, published in 1978, served as the model for the subsequent landmark Harvard Medical Practice Study. The California study had as its immediate goal "to obtain adequate information about patient disabilities resulting from health care management." This study did not use the term adverse event, but had as its focus the same idea, specifically "adverse outcomes to patients in the course of health care management," especially the subset of such outcomes consisting of potentially compensable events, namely disabilities caused by health care management. The California study reported 4.65 injuries to patients per 100 hospitalizations. Subsequent studies have consistently found that 10%–12% of patients experience harm while hospitalized, with approximately half of these events being considered preventable.

Definitions and Types of Patient Harm

Investigators in the Harvard Medical Practice Study defined an adverse event as "an injury that was caused by medical management (rather than the underlying disease) and that prolonged the hospitalization, produced a disability at the time of discharge, or both." The Institute for Healthcare Improvement uses a similar definition: "unintended physical injury resulting from or contributed to by medical care (including the absence of indicated medical treatment), that requires additional monitoring, treatment, or hospitalization, or that results in death."

Adverse events may be preventable or nonpreventable. One definition refers to preventable adverse events as "avoidable by any means currently available unless that means was not considered standard care." Preventable adverse events are defined as "care that fell below the standard expected of physicians in their community." These adverse events were the focus of both the Medical Insurance Feasibility Study and the Harvard Medical Practice Study. Examples of nonpreventable adverse events and preventable adverse events from the Harvard Medical Practice Study are provided in the Box.

Box. Example Adverse Event Cases From the Harvard Medical Practice Study

"Case 1: During angiography to evaluate coronary artery disease, a patient had an embolic cerebrovascular accident. The angiography was indicated and was performed in standard fashion, and the patient was not at high risk for a stroke. Although there was no substandard care, the stroke was probably the result of medical management. The event was considered adverse but not due to negligence."

"Case 4: A middle-aged man had rectal bleeding. The patient's physician completed only a limited sigmoidoscopy, which was negative. The patient had continued rectal bleeding but was reassured by the physician. Twenty-two months later, after a 14-kg (30 lb) weight loss, he was admitted to a hospital for evaluation. He was found to have colon cancer with metastases to the liver. The physicians who reviewed his medical record judged that proper diagnostic management might have discovered the cancer when it was still curable. They attributed the advanced disease to substandard medical care. The event was considered adverse and due to negligence."

In addition to preventable adverse events, two other terms appear commonly in the literature. Errors are defined as "an act of commission (doing something wrong) or omission (failing to do the right thing) leading to an undesirable outcome or significant potential for such an outcome." The related Systems Approach Patient Safety Primer discusses the relationship between errors and adverse events, summarized in the Swiss Cheese Model of accident causation. A near miss is defined as "any event that could have had adverse consequences but did not and was indistinguishable from fully fledged adverse events in all but outcome." (Some studies use the related terms "potential adverse event" and "close call.") In a near miss, an error was committed, but the patient did not experience clinical harm, either through early detection or sheer luck. For example, consider a patient who is admitted to the hospital and placed in a shared room. A nurse comes to administer his medications, but inadvertently gives his pills to the other patient in the room. The other patient recognizes that these are not his medications, does not take them, and alerts the nurse so that the medications can be given to the correct patient. This situation involved a high potential for harm, as a cognitively impaired or less aware patient may have taken the incorrect medications.

A final subcategory of adverse event is the ameliorable adverse event, a term first coined in a study of postdischarge of adverse events. Ameliorable adverse events are those that are not preventable, but the severity of the injury "could have been substantially reduced if different actions or procedures had been performed or followed." For instance, a patient with a new diagnosis of heart failure is discharged on furosemide (a diuretic) with a follow-up visit with a cardiologist in 4 weeks but no instructions for earlier follow-up or laboratory tests. Ten days later, the patient presents to the emergency department with acute kidney injury and critically low potassium. These adverse effects of diuresis are not preventable in themselves, but the severity could have been reduced by planning to have the patient come in for lab testing within a week of discharge.


Studies of the epidemiology of adverse events, such as a recent series of reports by the Office of the Inspector General, use a two-stage record review process in which patient charts are independently reviewed by two clinically experienced reviewers in order to determine whether an adverse event occurred and if so, whether it was preventable. It is important to note that even with highly trained reviewers, the level of agreement between reviewers with regard to the presence of an adverse event is usually only moderate. When an adverse event occurred, reviewers also may disagree about whether the event was preventable.

Designating an adverse event as preventable requires some judgment about the degree to which the evidence supports specific prevention strategies and the feasibility of implementing these strategies. As the science of patient safety advances, these judgments can change over time, such that more adverse events become regarded as preventable. For instance, after publication of the seminal paper on the central line bundle to prevent catheter-associated bloodstream infections, reviewers participating in adverse event studies might have begun to judge all central line–associated bloodstream infections as preventable.

Current Context

In summary, adverse events refer to harm from medical care rather than an underlying disease. Important subcategories of adverse events include:

  • Preventable adverse events: those that occurred due to error or failure to apply an accepted strategy for prevention;
  • Ameliorable adverse events: events that, while not preventable, could have been less harmful if care had been different;
  • Adverse events due to negligence: those that occurred due to care that falls below the standards expected of clinicians in the community.

Two other terms define hazards to patients that do not result in harm:

  • Near miss: an unsafe situation that is indistinguishable from a preventable adverse event except for the outcome. A patient is exposed to a hazardous situation, but does not experience harm either through luck or early detection.
  • Error: a broader term referring to any act of commission (doing something wrong) or omission (failing to do the right thing) that exposes patients to a potentially hazardous situation.
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