Emergency Department Data (ED)

Uses for Asthma Surveillance

ED data can be used to examine the statewide severity of asthma, both from the perspective of the community and from the perspective of individual persons. Some of the questions these data can answer are:

What are the number and rates of ED visits from asthma?

Are the asthma ED visit rates higher than the national rate or the HP2020 objectives?

Do Asthma ED visit rates vary by age, sex, race, and/or geography?

What are the yearly trends in asthma ED visit rates?

History of ED Data Collection at State and Local Level

Not all states collect ED data from hospitals1. These data are collected for billing and other administrative purposes, rather than for asthma surveillance. Some cities and counties have begun to pool ED data for the purpose of syndromic surveillance unrelated to asthma, yet these data also could serve as another source of information about ED asthma visits. For all of these ED databases, the data variables and the number of years that data are available vary by location.

Source of Data (How to Access the Data)

Ownership of these data also varies by location. In some states, the state health department has the legislative authority to collect the data; in other states, data are collected and maintained by a private organization that has agreements to share the data with the state health department. In some states, patient, provider, and hospital identifiers are not included in shared data sets.  A list of state hospital associations can be found on the website of the American Hospital Association at http://www.aha.org/about/srmassoc/index.shtml1. If state level data are not accessible, local health departments may be able to obtain data directly from their local hospitals; the American Hospital Association Web site also contains links to Metropolitan and Regional Associations and other health care associations.

Analysis Standards

The Council of State and Territorial Epidemiologists (CSTE) has jointly developed a standardized case classification to identify probable and possible asthma cases in hospital discharge data2. This standardized case classification can also be applied to ED asthma visits.

Confirmed Case:  There is no confirmed case classification for ED data.

Probable Case:  ED records listing asthma (any ICD-9-CM Code 493; any ICD-10-CM Code J45) as the primary discharge diagnosis. (Unlike the ICD codes for mortality, the ICD-9-CM does not include a code numbered J46).

Possible Case:  ED records listing asthma (any ICD-9-CM Code 493; any ICD-10-CM Code J45) as a secondary discharge diagnosis OR records for children younger than 12 years listing a primary discharge diagnosis of acute bronchitis and bronchiolitis (ICD-9-CM Code 466) or chronic bronchitis (ICD-9-CM Code 491.20, 491.21). (Although not specifically stated in the 1998 CSTE statement, acute bronchitis is represented by ICD-10-CM Code J20, acute bronchiolitis is ICD-10-CM Code J21, and chronic bronchitis is represented by ICD-10-CM Code J41-J42).

The 1998 CSTE statement suggests combining only the confirmed and probable cases for asthma case counts.  Asthma ED data are generally presented as rates per 10,000 population.  Standard demographic breakdowns used for analyzing state hospital discharge data also are used for ED visits and are summarized below.

Demographic Breakdowns

  • Age Categories: Rates can be calculated by age for single-year, 5-year, 10-year and infant age intervals.
  • Sex Categories: Rates can be calculated separately for “male” and “female” categories.
  • Race Categories: Rates can be calculated separately for “White,” “Black” and “Other” categories when such data are available.
  • Time Trends: Rates can be calculated by year if the cell size permits. Rates also can be calculated by month, day of week, and time of day.
  • Geographic Categories: Rates can be calculated at the state, county, city, and zip code levels if the cell sizes permit. Age standardized rates should be used to compare geographic units. It is also important when interpreting these rates to recognize that ED visit rates for the state may not be independent of county/city rates.

 NOTE:  Small sample size can result in the release or inferred release of confidential or sensitive information and can also affect reliability of rates. Please consider collapsing years or demographic groups, presenting confidence intervals, or suppressing rates and counts, or a combination of these, if sample size of the numerator or denominator is inadequate. In some cases, an aggregation of categories of data may be necessary to achieve the relative standard error of 30% that has been suggested to produce reliable rates (https://www.cdc.gov/nchs/data/statnt/statnt24.pdf).  For example, if the event count is <30 in any particular year, we suggest combining years to achieve numerators >20, and estimating trends based on 3-year rolling averages (e.g., calculate a single rate for 1978-1980, then 1979-1981, etc.).

National Indicators

ED visits for asthma are the basis of a national indicators for asthma in HP2020 (Objectives RD-3)3. These indicators use ED visits with a primary discharge diagnosis of asthma (any ICD-9-CM Code 493; any ICD-CM Code J45) for the numerator and resident population from the U.S. Census for the denominator. The objective focuses on a reduction of ED visits for asthma to:

  • 7/10,000 in children under age 5 years
  • 7/10,000 in children and adults age 5 to 64 years (Age adjusted to the year 2010 standard population)

National Comparison Data Source: National Hospital Ambulatory Medical Care Survey (NHAMCS), CDC, NCHS4.

Assessing Burden

In addition to calculating population-based ED visit rates, ED data also can be used to assess the relative burden of asthma on the ED system. Understanding the percent of ED visits accounted for by asthma can be useful to hospitals and help in developing and targeting interventions. Some states have unique identifiers and can use ED data to calculate relapse rates (e.g., return to ED within 72 hours, 2 weeks, or 1 month of the index

visit), cost of ED visits, percent of ED visits paid for by Medicaid or other sources, and to describe the co-morbidities of asthma ED visits. ED data could also be linked with other data sources to explore factors related to ED visits.

Limitations of These Data

  • These data represent the number of ED visits and not the number of people who visited the ED. Records do not include patient identifiers, thus there is no way to determine how many times a person went to the ED.  However, each ED visit is an adverse event of major consequences to the person, and thus the count and rate of total ED visits is an excellent reflection of the public health burden experienced by the community.
  • Practice patterns and payment mechanisms may affect decisions by health care providers to refer to the ED.
  • A state ED database may not be a complete census of ED visits of their residents. Residents who sought ED care in another state may not be reported in his or her state’s ED data set. Federal hospitals (military and veterans hospitals) are not generally included in state ED data sets; if a small geographic area includes a military base with a military hospital, then its rates may be artificially low. Furthermore, ED visits by out-of-state residents will be included in the data set. These data need to be included if calculating ED burden, but it is important to exclude these ED visits when calculating an ED visit rate for residents.
  • Often, data on race (and Hispanic ethnicity) are not reported by EDs/hospitals.

Federal and State Contacts and Resources for ED Data

Centers for Disease Control and Prevention (CDC)
National Center for Environmental Health
Asthma and Community Health Branch
General Number: (770) 488-3700

National Center for Health Statistics
Ambulatory Care Statistics Branch,
General Number:  (301) 458-4600

Research Data Center
General Number: (301) 458-4375


  1. States with Significant Hospital Data Collection Programs, http://www.aha.org/about/srmassoc/index.shtml
  2. CSTE Environmental and Chronic Disease Committees. Asthma Surveillance and Case Definition – CSTE Position Statement, 1998.  Available at: http://c.ymcdn.com/sites/www.cste.org/resource/resmgr/PS/1998-EHCD-1.pdf.
  3. Healthy People 2020. Available at: http://www.health.gov/healthypeople/
  4. McCaig LF, Ly N. National Hospital Ambulatory Medical Care Survey: 2000 Emergency Department summary. Advance Data from Vital and Health Statistics.  No 326. Hyattsville, MD: National Center for Health Statistics (NCHS), April 22, 2002.