National Surveillance of Emergency Department Visits for Outpatient Adverse Drug Events FREE

Outpatient use of drug therapies in the United States is common and may confer serious risks along with substantial therapeutic benefits.^1- 2 Historically, the public health burden of adverse events from therapeutic drug use among community-dwelling, nonhospitalized patients has been difficult to estimate, but the problem is large and can be expected to increase.^3- 5 In 2004, 82% of the US population reported using at least 1 prescription medication, over-the-counter medication, or dietary supplement in the previous week and 30% reported using 5 or more of these drugs.¹

Outpatient drug use will likely increase due to an aging population, the trend toward outpatient service delivery, the development of new prescription medications, the transition of prescription medications to over-the-counter availability, and the increasing use of drugs for chemoprevention. The recent implementation of the new Medicare prescription drug coverage benefit is designed to provide beneficiaries with additional financial support to help ensure their continued access to drug treatments,⁶ which may further increase outpatient drug use.

These trends underscore the need for ongoing surveillance of outpatient drug safety. Although much attention and effort have been directed to measuring, understanding, and preventing adverse drug events (ADEs) in hospitalized patients,^7- 8 less attention has been focused on ADEs occurring outside of health care facilities. This is due in part to the difficulty of obtaining timely, nationally representative surveillance data on outpatient ADEs.⁹

To enhance surveillance of outpatient drug safety, the Centers for Disease Control and Prevention (CDC), the US Consumer Product Safety Commission (CPSC), and the US Food and DrugAdministration (FDA) developed the National Electronic Injury Surveillance System–Cooperative Adverse Drug Event Surveillance project (NEISS-CADES). We report data from the first 2 years of NEISS-CADES to estimate and describe the national burden of ADEs that led to emergency department (ED) visits.

The 63 hospitals participating in the National Electronic Injury Surveillance System–All Injury Program (NEISS-AIP) are a nationally representative, stratified probability sample of all hospitals (excluding psychiatric and penal institutions) in the United States and its territories, with a minimum of 6 beds and a 24-hour ED.¹⁰ There are 4 size strata (very large, large, medium, and small) based on the number of annual ED visits and 1 children's hospital stratum.

The CPSC and CDC jointly train coders located at each hospital who review clinical records of every ED visit to identify all initial visits for injuries, poisonings, and ADEs. Approximately 500 000 injury-related ED visits are reported to NEISS-AIP each year. Participation by hospitals in NEISS-AIP is voluntary and confidentiality of all data is ensured by the Consumer Product Safety Act.¹¹ Data collection, management, quality assurance, and analyses were determined to be public health surveillance activities by CDC and FDA human subjects oversight bodies and therefore did not require human subject review or institutional review board approval.

For the Cooperative Adverse Drug Event Surveillance (CADES) component of NEISS-AIP, an adverse drug event case is defined as an incident ED visit for a condition that the treating physician explicitly attributed to the use of a drug or a drug-specific effect. Coders are instructed to examine the physician diagnoses recorded in the clinical record. If a condition is specifically linked to a drug in this section, then the case is included. If a diagnosis describes a condition commonly related to drug effects (eg, bleeding, hypoglycemia) coders examine other sections of the clinical record for evidence that the condition is, in fact, drug-related (eg, documentation of supratherapeutic international normalized ratio [INR] in a patient taking anticoagulants, documentation of insulin use in a patient with hypoglycemia).

Drugs include prescription or over-the-counter (OTC) medications; vaccines; and vitamins, dietary supplements, and herbal products. (In its organizations, operations, and regulations, the FDA distinguishes drug products from vaccines, vitamins, and dietary supplements. Drugs and vaccines require FDA approval before they can be sold; vitamins and dietary supplements do not.) Alcoholic beverages, tobacco products, and illicit substances are excluded.

Adverse events include allergic reactions (immunologically mediated effects)¹²; adverse effects (undesirable pharmacologic or idiosyncratic effects at recommended doses)¹²; unintentional overdoses (toxic effects linked to excess dose or impaired excretion)¹²; or secondary effects (eg, falls, choking). Intentional self-harm (eg, suicide attempts), drug therapeutic failures, drug withdrawal, and drug abuse are excluded. Adverse drug events that occur as a result of medical treatment received during the ED visit are excluded. Follow-up visits for an ADE previously diagnosed and treated are also excluded.

After identifying ADE cases, NEISS-AIP coders transcribe physician diagnoses and abstract from the clinical record the reason for visit, diagnostic tests, therapies administered, and the name, dose, route, frequency, and duration of use for up to 2 drugs associated with the adverse event. Coders also record up to 10 concomitant drugs as well as core NEISS-AIP data elements such as patient demographics and a narrative description of the incident. NEISS-AIP coders use a computer-based data entry system to transmit case reports to CPSC for initial quality review. Deidentified data are forwarded to CDC for further review and quality assurance. NEISS-AIP hospital coders and their supervisors receive specific instruction on identifying ADEs and abstracting additional data through training conferences, a coding handbook, electronic training materials, practice exercises, supplemental coding tools, and individual hospital reviews and site visits.^13- 14

In this study, an ED visit for an ADE was the primary outcome measure. A secondary clinical outcome was ADE severity as measured by the need for hospitalization following ED evaluation. Hospitalization includes admission to an inpatient unit of that facility, admission to the ED for observation, or transfer to another facility for acute medical care. NEISS data, including ADE data, are not used for national estimates of deaths. Details about event circumstances are often lacking when patients are dead on arrival or die soon after arrival in the ED, and such cases are incompletely captured across ED record systems. Therefore, deaths from ADEs occurring in the out-of-hospital setting, in the ED, or after hospital admission are not reported.

To describe the magnitude and epidemiology of ADEs treated in EDs, we classified ADEs by patient, event, and drug characteristics. Narrative summaries, clinical testing, and physician diagnoses were coded through FDA using the Medical Dictionary for Regulatory Activities (MedDRA) version 7.0 preferred terms, an international terminology used to analyze adverse event reports.¹⁵ Type of ADE and type of condition were categorized using the MedDRA terms describing diagnoses and symptoms.

Drugs were categorized by active ingredient and route of administration using the National Drug File Reference Terminology (acquired August 2003).¹⁶ Drugs not included in the National Drug File Reference Terminology (eg, certain nutritional supplements and OTC preparations) and drugs included in multiple classes were classified by the investigators. The following drugs were considered to commonly require regular monitoring because of a narrow therapeutic range: insulins, oral hypoglycemic agents, warfarin, digitalis glycosides, phenytoin, carbamazepine, divalproex, primidone, lithium, and theophylline.

Each NEISS-AIP case is assigned a sample weight based on the inverse probability of selection.¹⁰ These weights were summed and the sum divided by 2 to determine annual national estimates of ADEs in the period 2004-2005. Population ADE rates were calculated using 2004 and 2005 US population estimates from the US Census Bureau^17- 18 and were considered free of sampling error. Estimates and 95% confidence intervals (CIs) were calculated using the surveymeans procedure in SAS version 9.1 to account for the sample weights and complex sample design (SAS Institute Inc, Cary, NC). Estimates of less than 1200 individuals, based on fewer than 20 cases, or with coefficients of variation (CVs) greater than 30% may be unstable¹⁹ and are indicated in the tables.

Based on 21 298 ADE cases reported, we estimated that 701 547 US patients (95% CI, 509 642-893 452) were treated annually for ADEs in EDs in 2004 and 2005. Of these, 3487 case patients were hospitalized (annual estimate, 117 318 [16.7%]; 95% CI, 13.1%-20.3%). The hospitalized cases included 2932 admitted to an inpatient unit of the facility (annual estimate, 13.8%; 95% CI, 10.5%-17.1%), 385 held in the ED as observation admissions (annual estimate, 1.9%; 95% CI, 0.5%-3.4%), and 170 transferred to another health care facility (annual estimate, 1.0%; 95% CI, 0.7%-1.3%).

Based on 922 196 unintentional injury and ADE cases reported to NEISS-AIP, 2.5% (95% CI, 2.0%-3.1%) of estimated ED visits were due to ADEs and 6.7% (95% CI, 4.7%-8.7%) of estimated hospitalizations for unintentional injuries were due to ADEs (Table 1). Patients aged 65 years or older comprised 10.8% of all estimated unintentional injury visits (95% CI, 9.6%-12.1%) but 25.3% of estimated ADE visits (95% CI, 20.2%-30.4%). Patients aged 65 years or older accounted for 37.0% of estimated unintentional injury visits requiring hospitalization (95% CI, 31.8%-42.2%) but 48.9% of estimated ADE visits requiring hospitalization (95% CI, 40.0%-57.8%). While most ED visits for unintentional injuries overall were amongmen (annual estimate, 54.6%; 95% CI, 53.7%-55.5%), most ED visits for ADEs were among women (annual estimate, 60.6%; 95% CI, 59.1%-62.1%). Based on 4 451 726 total ED visits reported to NEISS (annual estimate, 120 490 979 visits), ADEs led to 0.6% of estimated ED visits for all causes.

The estimated annual population rate of ADEs treated in EDs was 2.4 per 1000 individuals (95% CI, 1.7-3.0). In infants and children younger than 5 years, the estimated annual rate of ADEs (4.3 per 1000; 95% CI, 3.1-5.4) was higher than the estimated annual rate for the general population rate but dropped among children aged 5 through 9 years (1.0 per 1000; 95% CI, 0.7-1.3) (Figure). The estimated annual rate of ADEs began to exceed the general population rate again in adults aged 60 through 64 years (2.9 per 1000; 95% CI, 1.9-3.9) and continued to increase with age until peaking at 6.8 per 1000 for adults aged 85 years or older (95% CI, 3.6-10.1). The estimated annual rate of ADEs for individuals aged 65 years or older was more than twice the rate for those younger than 65 years (4.9 per 1000; 95% CI, 2.7-7.0 vs 2.0 per 1000; 95% CI, 1.6-2.5) (rate ratio [RR], 2.4; 95% CI, 1.8-3.0). Overall, the estimated annual population rate of ADEs requiring subsequent hospitalization was 0.4 per 1000 (95% CI, 0.2-0.6). For persons aged 65 years or older, the estimated annual population rate of ADEs requiring hospitalization was nearly 7 times the rate for persons younger than 65 years (1.6 per 1000; 95% CI, 0.7-2.5 vs 0.23 per 1000; 95% CI, 0.15-0.31) (RR, 6.8; 95% CI, 4.3-9.2).

The most common conditions caused by ADEs were dermatologic, gastrointestinal, and neurological conditions (Table 2). Most adverse events manifested as a single type of condition (14 137 cases; annual estimate, 64.1%; 95% CI, 61.0%-67.4%). A quarter of adverse events involved 2 types of conditions (5353 cases; annual estimate, 26.6%; 95% CI, 25.0%-28.2%). Fewer adverse events involved 3 or more conditions (1808 cases; annual estimate, 9.3%; 95% CI, 7.0%-11.7%).

One third of estimated ED visits were attributed to allergic reactions (33.5%; 95% CI, 28.6%-38.4%), and one third were attributed to unintentional overdoses (32.1%; 95% CI, 28.6%-35.7%) (Table 3). Most of the estimated hospitalizations were attributed to unintentional overdoses (53.4%; 95% CI, 46.9%-59.9%), while allergic reactions accounted for 11.3% (95% CI, 6.1%-16.4%). The estimated proportion of patients hospitalized following unintentional overdoses (27.8%; 95% CI, 22.3%-33.3%) was 5 times greater than the estimated proportion hospitalized due to allergic reactions (5.6%; 95% CI, 3.5%-7.7%).

Drugs that commonly require regular outpatient monitoring to prevent acute toxicity (antidiabetic agents, warfarin, several anticonvulsants, digitalis glycosides, theophylline, and lithium) were involved in most unintentional overdoses (3387 cases; annual estimate, 53.3%; 95% CI, 41.6%-64.6%). These drugs were implicated in 66.0% of estimated overdoses requiring hospitalization (1149 cases; 95% CI, 53.8%-78.2%) and 41.5% of all estimated hospitalizations (1381 cases; 95% CI, 30.9%-52.1%). Among patients aged 65 years or older, these drugs that commonly require regular monitoring were implicated in 85.4% of estimated overdose visits (1744 cases; 95% CI, 80.3%-90.5%), 87.0% of estimated overdoses requiring hospitalization (708 cases; 95% CI, 82.3%-91.7%), and 54.4% of all estimated hospitalizations (829 cases; 95% CI, 45.0%-63.7%).

In 94.0% of estimated ADE visits, a single drug (18 315 cases; annual estimate, 86.6%) or drugs from the same therapeutic category (1611 cases; annual estimate, 7.4%) were implicated. The most common drug categories and classes implicated are listed in Table 4 with ADEs involving drugs from more than 1 therapeutic category listed separately. Overall, the 5 most common drug classes implicated in ADEs were insulins, opioid-containing analgesics, anticoagulants, amoxicillin-containing agents, and antihistamines/cold remedies. They accounted for 27.7% of estimated ADEs (5780 cases; 95% CI, 24.0%-31.3%). The 5 most common classes implicated in hospitalized ADEs were anticoagulants, insulins, opioid-containing analgesics, oral hypoglycemic agents, and antineoplastic agents, and these drug classes accounted for 38.4% of hospitalizations (1366 cases; 95% CI, 29.8%-46.9%).

Eighteen drugs were implicated alone or in combination with other drugs in 1% or more of estimated ADEs (Table 5). Insulins or warfarin, drugs that typically require ongoing monitoring to prevent overdose or toxicity, were implicated in 1 in every 7 estimated ADEs treated in EDs (14.1%; 95% CI, 9.6%-18.6%). Seven of these drugs were antibiotics, and together these antibiotics were implicated in 1 in every 8 estimated ADE treated in EDs (13.0%; 95% CI, 11.7%-13.3%). These 18 drugs were implicated in 44.6% of all estimated hospitalizations (1465 cases; 95% CI, 35.9%-53.2%). Insulin or warfarin was implicated in more than one quarter of all estimated hospitalizations (871 cases; 95% CI, 17.3%-35.2%) while the 7 antibiotics were implicated in only 3.8% of hospitalizations (121 cases; 95% CI, 2.7%-5.0%). Among patients aged 65 years or older, 3 drugs that typically require ongoing monitoring (insulin, warfarin, and digoxin) were implicated in 1 in every 3 estimated ADEs treated in EDs (1592 cases; 33.3%; 95% CI, 27.8%-38.7%) and 41.5% of estimated hospitalizations (646 cases; 95% CI, 32.4%-50.6%).

Based on data from a nationally representative surveillance system, we estimate that more than 700 000 patients were treated for ADEs in US EDs annually in 2004 and 2005, and 1 of every 6 required subsequent hospital admission, transfer to another health care facility, or ED observation admission. Individuals aged 65 years or older were more than twice as likely to be treated in EDs for an ADE and nearly 7 times as likely to require hospitalization as individuals younger than 65 years. Among all patients who were hospitalized, most ADEs were due to unintentional overdoses and two thirds of these were due to toxicity from a relatively small set of drugs for which regular monitoring is commonly required to prevent acute toxicity. Sixteen of the 18 drugs most commonly causing ADEs have been in clinical use for more than 20 years.²⁰

These population-based surveillance data help define the national scope of the outpatient ADE problem, underscore the need for intensified prevention efforts, and identify areas in which to focus interventions for the greatest public health impact. The finding that individuals aged 65 years or older (12% of the US population) accounted for one quarter of ADEs overall and half of adverse events requiring hospitalization highlights the importance of directing ADE prevention efforts to this vulnerable population. Emergency department visits for ADEs in this age group were nearly as common as those for motor vehicle occupant injuries.²¹

Important underlying factors contribute to the disproportionate effect of ADEs on individuals aged 65 years or older (eg, greater frequency and number of drugs used by this age group, age-related physiologic changes). However, the finding that just 3 drugs (warfarin, insulin, and digoxin), with narrow therapeutic index and high risk of overdose or toxicity, caused nearly one third of ED-treated ADEs in patients aged 65 years or older provides further focus for prevention efforts. A recent study found that high proportions of ambulatory patients taking drugs with a narrow therapeutic range had no serum concentration monitoring during 1 year of use.²² Other safety interventions designed to prevent these specific ADEs, such as patient education programs, patient self-management, and specialist management, have long been available but use of these interventions varies.²³ The data from our study emphasize the national scope of the adverse health outcomes due to outpatient ADEs that could be addressed through targeted implementation of current safety interventions.

We compared the magnitude of ADEs to the public health burden of unintentional injuries because an injury-oriented approach provides a valuable framework for understanding and preventing a wide range of harmful events.²⁴ Other unintentional injuries once considered unavoidable have been greatly reduced in frequency and severity using injury prevention techniques. Examples include reduction of motor vehicle–related injury with enforcement of speed limits²⁵ and installation of airbags,²⁶ reduction of needlestick injuries in health care workers with introduction of needle protective devices,²⁷ and reduction in unintentional overdose deaths after implementing requirements for child-resistant packaging.²⁸ Considering outpatient ADEs as an interaction of an agent (drug), a host (patient), and the environment (physical and social) can help conceptualize injury-based approaches for preventing or ameliorating outpatient ADEs.²⁹

The population-based surveillance data we report are consistent with findings from studies in single institutions,^30- 31 studies in Medicare enrollees,²³ and contention of drug safety analysts that a large proportion of the public health burden of ADEs is attributable to “older drugs, used poorly.”³² Direct comparisons between surveillance data from NEISS-CADES and previous reports on outpatient ADEs are limited by differences in case definitions and outcomes evaluated. For example, some ED-based studies have included drug abuse, suicide attempts, noncompliance or nonadherence, therapeutic failures of drugs or inadequate drug therapy, and adverse events from drugs given during ED visits.³³ Other studies of outpatient ADEs have measured a range of outcomes including patient self-reported symptoms,³⁴ potential ADEs,³⁵ and assessments of severity.³¹ Nevertheless, studies of single EDs³⁰ and studies of ADEs in a local Medicare population³⁶ have found similar types of adverse effects and implicated many of the same drugs and drug classes. Zhan et al³⁷ recently analyzed data from one source of nationally representative outpatient ADE data, the National Hospital Ambulatory Medical Care Survey (NHAMCS), and reported a similar rate of ED visits for ADEs (1.9 to 2.8 per 1000 for 1995 through 2001). However, this analysis was limited to pooled data for a 6-year study period to describe patient characteristics and broad drug categories based on International Classification of Diseases coding.

NEISS-CADES has several advantages compared with other sources of ADE information. Because NEISS-CADES collects data from a nationally representative sample of hospital EDs, data are not subject to the questions of generalizability as are data collected from single institutions or single geographic areas.³⁰ Moreover, compared with many research studies and population surveys, NEISS-CADES data collection is timely, with preliminary data typically available within several weeks of the ED visit.¹³ NEISS-CADES data collection is ongoing, enabling trend monitoring and evaluation of national safety interventions not possible with research studies that typically have defined study periods. In addition, although NEISS-CADES recently began operation, it will likely be less susceptible than voluntary reporting systems to variations in reporting rates over time,³⁸ just as the parent surveillance system, NEISS, has produced stable estimates of consumer product-related injuries for more than 3 decades.^19,39

Adverse drug event surveillance with NEISS-CADES has several limitations that likely result in an underestimate of the outpatient ADE burden. First, NEISS-CADES is restricted to ED patients. Cases of ADEs diagnosed and treated in other settings (eg, primary care offices, non–hospital-based urgent care centers, or directly admitted to hospitals) or not treated in any health care facility will not be captured.

Second, some ADEs that lead to ED visits, such as effects of chronic drug exposure, adverse effects manifested by the gradual onset of symptoms, and uncommon adverse effects, may be unrecognized by ED physicians and therefore may be undetected.

Third, because NEISS-CADES relies on documentation of ADEs by the treating physician, it is likely less sensitive than research studies involving chart review by specially trained pharmacists or physicians, computer-generated signals, patient interview, or combination approaches to identify undiagnosed or unreported ADEs.⁴⁰

Fourth, NEISS-CADES could be biased toward detecting acute, well-known drug effects or effects for which testing is available in the ED, such as hypoglycemia from insulin overdose or hypocoagulability due to warfarin. However, in an evaluation of 6 NEISS-CADES hospitals, these events were found to be underreported rather than overreported. Excluding these events improved the sensitivity of ADE identification from 33% to 45%.¹⁴ However, the weighted positive predictive value in this evaluation for coder-reported ADEs was 92%.¹⁴ Although this estimate of sensitivity for 6 of the participating hospitals may appear low, it is considerably higher than voluntary reporting, which often captures less than 1% of serious adverse reactions and rarely captures more than 10%.³⁸ Efforts to improve the sensitivity of ADE identification are part of the ongoing NEISS-CADES quality assurance process, and reassessments of ADE identification are planned.

And fifth, while ADE surveillance with NEISS-CADES provides information on outpatient ADEs treated in EDs, the NEISS-CADES data are insufficient to provide a complete perspective on outpatient ADEs. For instance, we did not estimate the total number of fatalities from outpatient ADEs because NEISS-CADES does not capture prehospital deaths or deaths in the ED. Moreover, although calculating the proportion of ADEs relative to that drug's use can provide information on relative risks for comparison with other drugs, we did not report such calculations because estimates of national outpatient drug use are not available from NEISS-CADES data. In addition, we did not attempt to categorize ADEs by “preventability” or the presence of a medication “error” in this investigation. Further examination of the data collected through this ED-based public health surveillance system is needed to determine if the clinical details are available for such categorization.

We expected national estimates of uncommon events (eg, estimates of <1200 per year or estimates based on fewer than 20 cases) to have CVs greater than 30%; however, several national estimates of ADE estimates were based on relatively large numbers of cases but also had CVs greater than 30% (eg, warfarin with 1234 cases, CV = 32.5%). This situation can occur for estimates generated from a probability sample, such as NEISS, when the underlying distribution of the condition under surveillance is not evenly distributed across the population. For example, NEISS estimates of ED visits for snow-skiing injuries have elevated CVs because hospitals in mountainous northern states have high numbers of visits for these injuries while hospitals in coastal southern states have low numbers of visits.

Drug selection,^41- 43 disease monitoring,^44- 45 therapeutic outcomes,⁴⁶ and adverse outcomes⁴⁷ have all been shown to vary by hospital and geographic area. These variations in clinical practice likely contribute to variation in the number of ADEs treated at NEISS hospitals, resulting in CVs greater than 30% for some estimates presented in this study. A recent systematic review reported that anticoagulation control varied extensively among study settings,⁴⁸ so it is not unreasonable that adverse events from anticoagulants might vary among the catchment areas served by NEISS hospitals. Other possible explanations for elevated CVs include variability in the level of detail in clinical documentation or variability in data collection across NEISS hospitals. Because ADEs are typically underdocumented,⁴⁰ and an evaluation of NEISS-CADES data collection found a high positive predictive value,¹⁴ variability due to documentation or data collection practices would likely increase these national estimates.

Efforts to reduce the burden of outpatient ADEs have been hampered by sparse data, except in selected health care systems or settings.³ Ongoing data collection in NEISS-CADES will enable more detailed examination of the epidemiology of ED-treated outpatient ADEs, focusing on specific patient populations, drug classes, conditions, and circumstances. Identifying appropriate measures of drug exposure and evaluating drug risks in relation to drug benefits remain important challenges in improving the quality of outpatient drug therapy. In the future, data from electronic health records may provide national, real-time data on outpatient drug safety.⁴⁰ Until then, leveraging existing public health surveillance systems provides a feasible, cost-efficient way to monitor the national health burden of outpatient drug safety problems and helps target prevention strategies tailored to the specific events of greatest population burden.