The following links provide data elements descriptions for the National (Nationwide) Inpatient Sample (NIS). Record layouts for the NIS files are available under File Specifications.

Data element descriptions explain how the data element is coded in the HCUP databases and what are the uniform values. The descriptions are cumulative across all years of NIS data from 1988 to the current data year. Please note the following:

• Not all data elements in the NIS are uniformly coded across States. Please check the "State Specific Notes" section for each data element before analysis.
• Not all data elements in the NIS are available from every State.
• Not all data elements are available each year.

For certain data elements such as, but not limited to, patient race and ethnicity (RACE) and total charges (TOTCHG), more than one percent of NIS records have missing values. Users are advised to review frequencies for all data elements planned for use in their analysis and to consider analytic methods for dealing with missing data. The HCUP Methods Series Report #2015-01 provides guidance on alternative approaches to handling missing data in the NIS.

NIS File Structure

Other than data year 2015, which was split into two parts because of the switch from ICD-9-CM to ICD-10-CM/PCS coding, the NIS is an annual, calendar year file. There are three discharge-level files and one hospital-level file:

NIS Discharge-level files

• Core File is a single file containing commonly used data elements (e.g., age, expected primary payer, ICD-10-CM/PCS codes, total charges).
  • This file is available in all years of the NIS.
  • Linkage between the discharge-level files
    • Prior to the 2012 NIS, the HCUP unique record identifier (KEY) provided the linkage between the discharge-level files.
    • Beginning with the 2012 NIS, the unique NIS record number (KEY_NIS) provides the linkage between the discharge-level files.
• Severity File is a single file containing additional data elements to aid in identifying the severity of the condition for a specific discharge.
  • This file is available beginning with the 2002 NIS.
• Diagnosis and Procedure Groups File is a single file containing additional information on the ICD-10-CM diagnoses and ICD-10-PCS procedures that is created by the Agency for Healthcare Research and Quality (AHRQ) software tools.
  • This file is available beginning with the 2005 NIS.
  • For data years 2016-2017, this file was not available in the NIS. Data elements derived from the ICD-10-CM/PCS AHRQ software tools were not included in the NIS because they were still in development and testing.
  • Beginning with data year 2018, this file is available in the NIS and includes data elements derived from the Clinical Classifications Software Refined (CCSR) for ICD-10-CM diagnoses.
  • Beginning with data year 2019, data elements derived from the Elixhauser Comorbidity Software Refined for ICD-10-CM, the CCSR for ICD-10-PCS procedures, and Procedure Classes Refined for ICD-10-CM are also available in this file.
  • Beginning with data year 2023, data elements derived from the Chronic Conditions Indicator Refined (CCIR) for ICD-10-CM are available for the file.

NIS Hospital-level file

• Hospital File is a single file containing information on hospital characteristics.
  • This file is available in all years of the NIS.
  • Linkage between the Inpatient Core File and the Hospital File
    • Prior to the 2012 NIS, the HCUP hospital identifier (HOSPID) provided the linkage between the NIS Inpatient Core File and the Hospital File.
    • Beginning with the 2012 NIS, the NIS hospital number (HOSP_NIS) provides the linkage between the NIS Inpatient Core File and the Hospital File. The HOSP_NIS values are reassigned each year, so they cannot be used to link hospitals across years.

Users interested in applying AHRQ software tools to the NIS for data years including ICD-10-CM/PCS-coded data to produce data elements currently unavailable in the database files may do so by downloading the respective tool(s) from the Research Tools section of the HCUP User Support (HCUP-US) website. Additionally, users may wish to review the HCUP Software Tools Tutorial, which provides instructions on how to apply the AHRQ software tools to HCUP or other administrative databases.

For information on changes to the NIS in data year 2015, refer to the document 2015 Healthcare Cost and Utilization Project (HCUP) National Inpatient Sample: Change in Structure and Data Elements Caused by Transition to ICD-10-CM/PCS.

All NIS data elements are listed in alphabetical order below, showing the NIS file location for years other than 2015 and for 2015. For descriptions of HCUP data elements uniform values, check the corresponding boxes below and then click "View Description of Data Elements".