KEY contains a unique record identifier. Beginning in the 1998 data, all HCUP databases are sorted by KEY.

KEY can be used to link within a HCUP database, such as linking records in the Core and Charges files in the SID.

KEY can be used to link across HCUP databases within a data type, i.e., link records in the SID to records in the NIS.

KEY is a unique record identifier and not a person identifier. KEY cannot be used to link records between HCUP inpatient and ambulatory surgery files.

KEY replaces the database-specific record identifiers used in the 1988-1997 HCUP databases (SEQ, SEQ_SID, and SEQ_ASD).

KEY is included as the unique record number for the 2000 KID. RECNUM is the unique record number for the 1997 KID, 2003 KID, and subsequent years.

The length of the value of KEY was increased from 14 characters to 15 characters starting in the 2011 HCUP databases. KEY is loaded into SAS, SPSS, and Stata as a numeric data element.