Saul N Weingart, associate physiciana, Ross McL Wilson, senior specialist intensive careb, Robert W Gibberd, associate
professorc, Bernadette Harrison,
managerb.
a Division of General Medicine and Primary Care, Beth Israel
Deaconess Medical Center, 330 Brookline Avenue, Boston, MA 02215, USA, b Quality
Assurance Royal North Shore, Royal North Shore Hospital, St Leonards, NSW
2065 Australia, c Department of Statistics, University of Newcastle,
Newcastle, NSW 2308 Australia
Newspaper and television stories of catastrophic injuries occurring at the
hands of clinicians spotlight the problem of medicalerror but
provide little insight into its nature or magnitude.1Clinicians, patients, and policymakers may underestimate the magnitudeof risk and the extent of harm. We review the epidemiology of
medical error, concentrating primarily on the prevalence and consequencesof error, which types are most common, which clinicians make errors,and the risk factors that increase the likelihood of injury fromerror.
Summary points
The Harvard and Australian studies into medical error remain the only
studies that provide population level data on the rates of injuries to
patients in hospitals and they identified a substantial amount of medical
error
In the United States medical error results in 44 000-98 000 unnecessary
deaths each year and 1 000 000 excess injuries
Errors often occur when clinicians are inexperienced and new procedures
are introduced
Extremes of age, complex care, urgent care, and a prolonged hospital
stay are associated with more errors
Prevalence and consequences in hospitals
Benchmark studies
The Harvard study of medical practice is thebenchmark for estimating
the extent of medical injuries occurringin hospitals. Brennan et al
reviewed the medical charts of 30121 patients admitted to 51 acute
care hospitals in New York statein 1984.2
They reported that adverse eventsinjuries
causedby medical management that prolonged admission or produced
disabilityat the time of dischargeoccurred
in 3.7% of admissions. A subsequentanalysis of the same data found
that 69% of injuries were causedby errors.3
In a study of the quality of Australian health care, a population based study
modelled on the Harvard study, investigatorsreviewed the medical
records of 14 179 admissions to 28 hospitalsin New South Wales and
South Australia in 1995.4 An adverse
event occurred in 16.6% of admissions, resulting in permanent
disability in 13.7% of patients and death in 4.9%; 51% of adverse
events were considered to have been preventable. The number of
preventable adverse events is important because both preventableand
potential adverse events (or "near misses") imply medicalerror. In
contrast, non-preventable adverse events suggest thatanticipated and
unavoidable "complications" were present. In theAustralian study the
higher rate of adverse events was attributedin part to
methodological differences between the two studies,but a real
difference in the rate of injuries to patients in thetwo populations
could not beexcluded.
No study rivals the scope of the Harvard study and the Australian study
except for a recent replication of the Harvard studyin Colorado and
Utah.5 Even so, the results probably representan estimate of the lower boundary of the prevalence of medical
injury and error. The Harvard investigators defined adverse events
stringently, using disability and injury as prerequisites. This
underestimates the error rate as many errors don't produce injury
because they are caught in time, the patient is resilient, orbecause
of goodluck.
Beyond chart review
Aggressive case finding may identify injuriesand errors that are not
documented in a patient's chart.6 Using
a computerised model to detect adverse drug events among patientsat
a hospital in Salt Lake City, Utah, Classen et al found thatadverse
drug events occurred in 1.7% of admissions.7 In
comparison,using both chart review and prompted self reports from
clinicians,Bates et al found that adverse drug events occurred among
6.5%of patients and potential adverse drug events occurred among
5.5%of patients admitted to two teaching hospitals in Boston.8Of the adverse drug events 28% were due to errors, making the
rate of serious medication errors (that is, preventable adversedrug
events plus potential adverse drug events) 7.3%. Both ofthese
studies reported rates that are much higher than the 0.7%rate of
adverse drug events identified in the Harvard study'sreview of
medicalrecords.
Observational studies, although costly, have identified even higher rates of
error and injury occurring during medical care.For example,
observers on the general surgical units of a Chicagoteaching
hospital who recorded all "situations in which an inappropriate
decision was made when, at the time, an appropriate alternativecould
have been chosen" found that 45.8% of patients experiencedan adverse
event.9 Eighteen per cent of these patients hada "serious" adverse eventthat
is, one that produced at leasttemporarydisability.
Similarly, Donchin et al placed an observer at the patient's bedside to
observe clinicians in the medical-surgical intensivecare unit of a
university hospital in Israel. Clinicians made554 errors over four
months, or 1.7 errors per patient per day.10
Patients injured as a result of a medical error spend longer in hospital and
have higher hospital costs. At the hospital inUtah adverse drug
events caused complications in 2.4% of admissions,cost an average of
$2262 (£1414) per patient, and increased thelength of stay by
1.9 days in comparison with data from matchedcontrols.11
In the Harvard study of adverse drug events, theincremental cost
associated with an event was $2595 and the lengthof stay was
increased by 2.2 days. However, among preventableadverse drug
events, the excess cost was $4685 and the lengthof stay was
increased by 4.6 days.12 The cost of adverse drugevents for a 700 bed teaching hospital was estimated to be $5.6m(£3.5m) ayear.
Thus medical error is ubiquitous and the costs are substantial. In Australia
medical error results in as many as 18 000 unnecessarydeaths, and
more than 50 000 patients become disabled each year.In the United
States medical error results in at least 44 000(and perhaps as many
as 98 000) unnecessary deaths each year and1 000 000 excess
injuries.13
Prevalence and consequences among
outpatients
Comparatively little is known about the prevalence of medical error outside
hospitals. In both the Harvard study and the Australianstudy 8-9% of
adverse events occurred in a doctor's office, 2-3%at home, and 1-2%
in nursing homes. In the Australian study abouta quarter of the
adverse events occurring among outpatients causedpermanent
disability or death, and investigators judged it likelythat more
than two thirds could have been prevented. In otherstudies
iatrogenic injury accounted for 5-36% of admissions tomedical
services14-16 and 11-13% of adult admissions to
intensivecare units at several university hospitals.17-19
Since these studiesincluded only errors that were serious enough to
require admission,these figures underestimate the extent of error
associated withoutpatientcare.
Data on risk management also underestimate medical error occurring among
outpatients because there is little association betweenmalpractice
claims and medical error.20 Asking clinicians andpatients about errors provides information that is more useful.
Burnum reported that in a series of 1000 consecutive patientsseen in
his three person internal medicine practice 42 adversedrug reactions
occurred, of which 10 were preventable.21 Morerecently, Bhasale et al collected anonymous incident reports fromAustralian general practitioners of "an unintended event . . .that
could have harmed or did harm a patient."22 Of
805 incidentsinvolving drug treatment, diagnosis, and equipment, 27%
had thepotential to cause severe harm and 76% were judged to have
beenpreventable.
In the most rigorous outpatient study, Gandhi et al evaluated complications
associated with medications among patients at11 primary care sites
in Boston.23 Of 2258 patients who had
had drugs prescribed, 18% reported having had a drug related complication,such as gastrointestinal symptoms, sleep disturbance, or fatigue,in the previousyear.
To calculate the cost of drug related morbidity and mortality among
outpatients in the United States Johnson and Bootman asked
pharmacists to estimate the probability of adverse outcomes occurring
as a result of drug treatment.24 The costs
associated with adverseoutcomes were taken from statistical and
research reports. Theauthors calculated that drug related problems
accounted for 116million extra visits to the doctor per year,
76 million additionalprescriptions, 17 million emergency department
visits, 8 millionadmissions to hospital, 3 million admissions to
long term carefacilities, and 199 000 additional deaths. The total
cost wasestimated to be $76.6bn, rivalling the aggregate cost of
caringfor patients withdiabetes.
Types of medical error
In both the Harvard study and the Australian study about half of the adverse
events occurring among inpatients resulted fromsurgery.
Complications from drug treatment, therapeutic mishaps,and
diagnostic errors were the most common non-operative events.In the
Australian study cognitive errors, such as making an incorrect
diagnosis or choosing the wrong medication, were more likely tohave
been preventable and more likely to result in permanent disability
than technical errors.25
Adverse drug events have been investigated extensively because they are
prevalent and preventable. In Bates et al's studyof adverse drug
events at two teaching hospitals in Boston, 1%of the events were
fatal, 12% were life threatening, 30% wereserious, and 57% were
significant.8 Forty two per cent of the
adverse events classed as life threatening or serious were preventable.Errors resulting in preventable adverse events occurred most oftenduring ordering (56%) and administering (24%). Altogether, 245of
the near misses were the result of ordering errors and 40%were the
result of errors in administering drugs. Adverse eventswere
associated with the use of analgesics, antibiotics, sedatives,
chemotherapeutic agents, cardiovascular drugs, and anticoagulants.8
Missed and delayed diagnoses may be difficult to detect retrospectively by
chart review unless patients continue to use thesame sources of
care. In the Australian study errors of omissionoutnumbered errors
of commission by 2 to 1. An error of omissionis a failure of action
such as a missed diagnosis, a delayed evaluation,or a failure to
prescribe needed drug treatment. An error of commissionis an
incorrect action, such as administering the wrong drug tothe wrong
patient at the wrongtime.
Necropsy studies can also uncover missed diagnoses and misdiagnoses. A study
of 61 patients who died in a Spanish emergencydepartment identified
unexpected major findings, such as malignanttumours or haemorrhagic
pancreatitis, in 44% of cases and importantdiscrepancies between the
necropsy report and the clinical diagnosisin 26%.26
In a retrospective study of 524 deaths occurring in1990-1 at a
Spanish tertiary care hospital, more than half ofthe risk of death
was accounted for by adverse events that resultedfrom clinical care.27
The number of misdiagnoses and major unexpectedfindings at necropsy
has remained essentially unchanged for over40 years, prompting
healthcare leaders to cite the falling rateof necropsy as an
important impediment to ensuring the safetyof patients.28
Clinicians who make errors
Despite rare examples of malevolent providers, there is little evidence that
much medical error is due to "bad apples."29Although anaesthesiologists pioneered modern research into the
safety of patients, no specialty is immune to error.30
Proceduralmishaps are common in surgical specialties, perhaps
because theyare hard to disguise. Mistakes may be more common when
the clinicianis inexperienced and when new techniques are
introduced.31 Misreadradiographs and
pathology specimens,32 laboratory errors,33and mistakes made in administering radiation therapy also threatenthe safety of patients.34
Trainees often err. Wu et al surveyed medical house officers in three
training programmes in internal medicine about theirmost serious
mistake.35 Altogether, 45% reported making at
leastone error, 31% of which resulted in a patient's death. Lesar etal found that more prescribing errors occurred among first year
postgraduate residents than among other clinicians.36
Wilsonet al found that more errors occurred in a paediatric
intensivecare unit when new doctors joined the rotation.37
Risk factors for injury
A potentially attractive strategy for preventing injury from medical error is
to identify which patients are at an increasedrisk of harm. Patients
aged over 64, for example, have a greaterrisk of serious injury from
adverse events than younger patients. 2478 This may reflect their greater burden of comorbid illness andfrailty (figure).
Number of adverse events, preventable
adverse events, and number resulting in permanent disability by age.
Adapted from the quality in Australian healthcare study4
Certain interventions signal that there is a high risk, such as
cardiothoracic surgery, vascular surgery, and neurosurgery.The
severity of the patient's underlying illnesses as well asthe
inherent hazards of certain procedures may increase the likelihoodof
poor outcomes. Wilson et al found that a greater risk of deathand a
greater number of preventable adverse events were associatedwith
patients with complex cases, illnesses requiring urgent care,and the
use of interventions thought to be potentially life saving.4
Being cared for in the emergency department causes many preventable adverse
events. 2438 Several factors are implicated:the
use of part time doctors not trained in emergency care; fluctuating
demand for services, which results in uneven and sometimes abbreviatedcare; the limited time available to arrive at a definitive diagnosis;and the fact that the emergency department is the point of entryfor acutely illpatients.
The characteristics of individual patients may be less important than the
duration of care in explaining injury. Andrews etal reported that
the likelihood of an adverse event increasedby 6% for each day spent
in hospital.9 The intensity of carealso
affects the risk of injury. Among paediatric patients admittedto a
British university hospital, drug errors were seven timesmore likely
to occur in the intensive care unit than elsewhere.37Similarly, the Harvard investigators looking at adverse drug eventsfound that these events occurred more often among adult patientsin medical intensive care units than in surgical intensive care
units or general medical and surgical wards.39
However, whenthe number of doses dispensed in the different units
was adjustedfor, the differences were not significant. In a
subsequent analysisof the study, no independent risk factors for
preventable adverseevents were identified after the length of stay
before the eventand the use of intensive care were controlledfor.
Discussion
Although researchers regularly publish studies of medical error, adequate
epidemiological information is limited to a fewinstitutions,
procedures, and specialties. Because most studieswere conducted in
academic referral centres the results may notbe generalisable to
community based hospitals and outpatient carefacilities.
Comparing studies is difficult because research methods are not standardised.
The lack of agreement about methods and thevariable rigour of their
application contribute to the variationsfound in error rates. There
is a serious need for researchersto use consistent definitions and
methods and for collaborativework on measuringerror.
Systems for monitoring and reporting error could provide the platform from
which more detailed studies of subpopulations coulddevelop. However,
expecting that individuals will carry out healthcare flawlessly
creates an environment in which clinicians arereluctant to report
their errors. Universal underreporting, inturn, undermines the
ability to measure erroraccurately.
For these reasons the precise prevalence and magnitude of medical error is
unknown, but it is probably enormous. We are awareof no study
showing that medical care can be provided withouterror. In fact, the
more closely we examine patient care, themore error we find. No
setting is free from hazards and no specialtyis immune, and patients
are at risk no matter what their age,sex, or health
status.
But the risk is not homogeneous. Patients who are sicker, subjected to
multiple interventions, and who remain in hospitallonger are more
likely to suffer serious injury as a result ofmedical mistakes.
Unless we make substantial changes in the organisationand delivery
of health care, all patientsparticularly
the mostvulnerablewill
continue to bear the burden of medicalerror.
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-- Albert Einstein, letter to a friend, 1901
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