An act of commission (doing something wrong) or omission (failing to do the right thing) that leads to an undesirable outcome or significant potential for such an outcome. For instance, ordering a medication for a patient with a documented allergy to that medication would be an act of commission. Failing to prescribe a proven medication with major benefits for an eligible patient (e.g., low-dose unfractionated heparin as venous thromboembolism prophylaxis for a patient after hip replacement surgery) would represent an error of omission.

Errors of omission are more difficult to recognize than errors of commission but likely represent a larger problem. In other words, there are likely many more instances in which the provision of additional diagnostic, therapeutic, or preventive modalities would have improved care than there are instances in which the care provided quite literally should not have been provided. In many ways, this point echoes the generally agreed-upon view in the health care quality literature that underuse far exceeds overuse, even though the latter historically received greater attention. (See definition for Underuse, Overuse, Misuse.) In addition to commission vs. omission, three other dichotomies commonly appear in the literature on errors: active failures vs. latent conditions, errors at the sharp end vs. errors at the blunt end, and slips vs. mistakes.

Error Chain
Error chain generally refers to the series of events that led to a disastrous outcome, typically uncovered by a root cause analysis. Sometimes the chain metaphor carries the added sense of inexorability, as many of the causes are tightly coupled, such that one problem begets the next. A more specific meaning of error chain, especially when used in the phrase "break the error chain," relates to the common themes or categories of causes that emerge from root cause analyses. These categories go by different names in different settings, but they generally include (1) failure to follow standard operating procedures, (2) poor leadership, (3) breakdowns in communication or teamwork, (4) overlooking or ignoring individual fallibility, and (5) losing track of objectives. Used in this way, "break the error chain" is shorthand for an approach in which team members continually address these links as a crisis or routine situation unfolds. The checklists that are included in teamwork training programs have categories corresponding to these common links in the error chain (e.g., establish a team leader, assign roles and responsibilities, and monitor your teammates).

Use of the phrase "evidence-based" in connection with an assertion about some aspect of medical care—a recommended treatment, the cause of some condition, or the best way to diagnose it—implies that the assertion reflects the results of medical research, as opposed to, for example, a personal opinion (plausible or widespread as that opinion might be). Given the volume of medical research and the not-infrequent occurrence of conflicting results from different studies addressing the same question, the phrase "reflects the results of medical research" should be clarified as "reflects the preponderance of results from relevant studies of good methodological quality."

The concept of evidence-based treatments has particular relevance to patient safety, because many recommended methods for measuring and improving safety problems have been drawn from other high-risk industries, without any studies to confirm that these strategies work well in health care (or, in many cases, that they work well in the original industry). The lack of evidence supporting widely recommended (sometimes even mandated) patient safety practices contrasts sharply with the rest of clinical medicine. While individual practitioners may employ diagnostic tests or administer treatments of unproven value, professional organizations typically do not endorse such aspects of care until well-designed studies demonstrate that these diagnostic or treatment strategies confer net benefit to patients (i.e., until they become evidence-based). Certainly, diagnostic and therapeutic processes do not become standard of care or in any way mandated until they have undergone rigorous evaluation in well-designed studies.

In patient safety, by contrast, patient safety goals established at state and national levels (sometimes even mandated by regulatory agencies or by law) often reflect ideas that have undergone little or no empiric evaluation. Just as in clinical medicine, promising safety strategies sometimes can turn out to confer no benefit or even create new problems—hence the need for rigorous evaluations of candidate patient safety strategies just as in other areas of medicine. That said, just how high to set the bar for the evidence required to justify actively disseminating patient safety and quality improvement strategies is a subject that has received considerable attention in recent years. Some leading thinkers in patient safety argue that an evidence bar comparable to that used in more traditional clinical medicine would be too high, given the difficulty of studying complex social systems such as hospitals and clinics, and the high costs of studying interventions such as rapid response teams or computerized order entry.

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