There are up to three different types of hospital identifiers included in the HCUP databases.

• The data source's own number scheme for identifying hospitals and facilities (DSHOSPID),
• The hospital identifier used by the American Hospital Association (AHAID and IDNUMBER), and
• A unique HCUP hospital identifier (HOSPID).

The hospital entity as defined by the data source may differ from the hospital entity as defined by the American Hospital Association (AHA). For example, the data source treats two separate facilities as two hospitals, while the AHA treats the two facilities as a single hospital, or vice versa. For consistency across states, HCUP defines hospitals in accordance with the AHA Annual Survey Database (Health Forum, LLC © 2007). During HCUP data processing, the data source's identification of the hospital is reconciled with the identification of the hospital in the AHA Annual Survey Database. For detailed information about this linking process, see the special report on HCUP Hospital Identifiers.

The hospital identifier (AHAID) contains the 7-digit hospital identifier used on the AHA Annual Survey Database. These files contain information about hospital characteristics and are available for purchase through the AHA.

AHAID is missing if the data source that contributed discharge data to the NIS prohibits the release of hospital identifiers. Maps indicating the availability of the AHAID in the NIS provide detailed information regarding which States publicly release this information by year