The Nationwide Ambulatory Surgery Sample (NASS) is part of a family of databases and software tools developed for the Healthcare Cost and Utilization Project (HCUP).

The NASS is the only all-payer ambulatory surgery database in the United States, yielding national estimates of selected therapeutic ambulatory surgery encounters performed in hospital-owned facilities. Unweighted, the NASS contains approximately 9 million ambulatory surgery encounters and approximately 11 million ambulatory surgery procedures each year. Weighted, it estimates approximately 13 million ambulatory surgery encounters and 18 million ambulatory surgery procedures. Developed through a Federal-State-Industry partnership sponsored by the Agency for Healthcare Research and Quality, HCUP data inform decision-making at the national, State, and community levels.

This page provides an overview of the AHRQ HCUP NASS. For more details, see NASS Database Documentation and the Introduction to the NASS, for 2023 (PDF file, 1.2 MB).

Contents:

• What's New in the 2023 AHRQ HCUP NASS?
• About the AHRQ HCUP NASS
• AHRQ HCUP NASS File Structure
• AHRQ HCUP NASS Data Elements
• Areas of Research and HCUP Publications
• Multi-Year or Trend Analyses
• Purchase the AHRQ HCUP NASS
• NASS Hardware and Software Requirements

The total number of in-scope ambulatory surgery encounters for data year 2023 increases by 9 percent from 12.4 million in 2022 to 13.5 million.

Because of a change in the states available to participate in the 2023 NASS and a need to produce accurate national estimates, the following modifications have been made to the data elements included in the NASS:

• Remove information identifying the Census region of the hospital:
  • Remove the data elements that identified Census region (HOSP_REGION).
  • Revise the coding of the data elements for the hospital identifier (HOSP_NASS) and record identifier (KEY_NASS) to remove the information identifying Census region.
• Limit the information released on patient characteristics:
  • Remove the data element identifying the patient's race and ethnicity (RACE).
  • Replace the data element identifying the detailed metro status designation of the county of the patient's residence (PL_NCHS) with a new consolidated data element that distinguishes only two categories: metropolitan and non-metropolitan (PL_NCHS2).
• Remove the data element for total hospital charge (TOTCHG) due to lack of information needed to make adjustments for missing data.
• Changes to the Diagnosis and Procedure Groups file:
  • Add data elements derived from the Chronic Condition Indicator Refined (CCIR) for ICD-10-CM, v2025.1.
  • Used v2025.1 for all HCUP software tools included in the Diagnosis and Procedures Group file.
• For more information on NASS data elements, see the NASS Description of Data Elements.
• Please note that these modifications to the 2023 NASS data elements may make comparisons of some estimates across years more difficult. See the Introduction to the NASS, for 2023 (PDF) for more details.

Sampled from the State Ambulatory Surgery and Services Databases (SASD), NASS can be used to create national estimates of in-scope ambulatory surgery encounters performed in hospital-owned facilities. In-scope ambulatory surgeries are defined as therapeutic surgical procedures that typically require the use of an operating room, penetrate or break the skin, and involve regional anesthesia, general anesthesia, or sedation to control pain (i.e., surgeries flagged as "narrow" in the HCUP Surgery Flag Software). To be considered in-scope, ambulatory surgeries are also required to have a relatively high annual volume or aggregate total facility charge. Examples include ambulatory surgeries such as cataract surgery, cholecystectomy, appendectomy, gastric bypass, hysterectomy, hernia repair, spinal fusion, and hip replacement.

NASS data are available from 2016 through 2023, which allows researchers to analyze trends over time.

For more details on the 2023 NASS, see the Introduction to the NASS, 2023 (PDF file, 1.2 MB).

Information on previous years of the NASS may be found in prior years of the Introduction to the NASS at https://hcup-us.ahrq.gov/db/nation/nass/nassarchive.jsp.

The NASS is an annual, calendar year file. There are three encounter-level files and one hospital-level file:

NASS Encounter-level Files

• Encounter File contains one record for all ambulatory surgery encounters containing an in-scope procedure from hospital-owned facilities in participating States and the District of Columbia.
• Supplemental File contains one record with information on not-in-scope procedures that were performed during encounters recorded in the Encounter File. This file includes fewer records than the Encounter File because not all encounters included both in-scope and not-in-scope procedures. If an encounter had no not-in-scope procedures, then there is no record in the Supplemental File.
  • The unique NASS record identifier (KEY_NASS) provides the linkage between the NASS Encounter File and the Supplemental File.
• Diagnosis and Procedure Groups File contains one record for each record in the Encounter File with additional information on ICD-10-CM diagnoses, derived from the AHRQ HCUP software tools.
  • The unique NASS record identifier (KEY_NASS) provides the linkage between the NASS Encounter File and the Diagnosis and Procedure Groups File.

NASS Hospital-level Files

• Hospital File contains one observation for each hospital-owned facility included in the NASS, along with encounter weight and select hospital characteristics.
  • The unique NASS hospital identifier (HOSP_NASS) provides the linkage between the NASS Encounter File and the Hospital File.

Users interested in applying AHRQ software tools to the NASS 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.

The NASS contains clinical and resource-use information that is included in a typical hospital-owned facility record abstract, including patient characteristics, clinical diagnostic and surgical procedure codes, total charges and expected source of payment, and facility characteristics. Safeguards are applied to protect the privacy of individual patients, physicians, and hospitals (as required by data sources). The NASS contains clinical and nonclinical data elements for each encounter. These include:

• Current Procedural Terminology (CPT®) procedure codes for encounters with at least one in-scope ambulatory surgery
• International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) diagnosis codes
• Data elements derived from the AHRQ software tools
• Patient demographic characteristics (e.g., sex, age, race and ethnicity, urban-rural designation of residence, national quartile of median household income for patient's ZIP Code)
• Expected payment source (e.g., Medicare, Medicaid, private insurance, self-pay)
• Total charges (not available in 2023 NASS)
• Disposition of the patient
• Hospital characteristics (e.g., ownership, teaching status)

Not all data elements are available for every year. For comprehensive information about the data elements and their availability by data year, see the NASS Description of Data Elements.

For information on modifications to the NASS data elements in data year 2023 due to changes in the participating states relative to prior years of the NASS, see the Introduction to the NASS, for 2023 (PDF) and the NASS Description of Data Elements.

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As a uniform, multi-State database, the NASS promotes wide-ranging studies of ambulatory surgeries in hospital-owned facilities and supports health care policy and research on a variety of topics, including:

• Trends in ambulatory surgery
• Charges for ambulatory surgery services
• Quality of ambulatory surgery care
• Impact of health policy changes
• Access to ambulatory surgery care
• Utilization of ambulatory surgery services by special populations

The NASS may be used in a variety of publications:

• HCUP Statistical Briefs highlight a variety of health topics.
• Use the HCUP Publications Search Tool to find publications using the NASS.
• Review featured publications on the HCUP Research Spotlights page.
• Read publications by the winners of the HCUP Outstanding Article of the Year Awards.

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When conducting longitudinal analyses, users should exercise caution and consider several aspects of the NASS design and changes to the design over time.

• Procedures considered in scope for the NASS sample can change from year to year. These changes may result from an increase or decrease in the volume of procedures performed in the outpatient setting, as this determines whether a Clinical Classifications Software for Services and Procedures (CCS-Services and Procedures) category meets the threshold for inclusion in the NASS sample. Additionally, the 2018 NASS applied updated versions of the HCUP Surgery Flag Software for Services and Procedures (that expanded the range of possible CPT codes) and the CCS-Services and Procedures Tool. Combined, these updates contributed to changes in the CPT procedures and CCS-Services and Procedures procedure groups defined as in-scope for the NASS sample. The NASS in-scope procedure criteria also changed between data years 2018 and 2019, which resulted in some changes to the CCS-Services and Procedure categories considered in scope for the NASS.
• The sampling of the NASS changed over time to capture emergent in-scope ambulatory surgeries (i.e., in-scope narrow surgeries that were started in the emergency department). Starting with data year 2020, the NASS was sampled only from the SASD because records for emergent in-scope ambulatory surgeries were included in the SASD. In data year 2019, the NASS was sampled from the SASD and State Emergency Department Databases (SEDD) because records for emergent in-scope ambulatory surgeries were only included in the SEDD (and not in the SASD). In the 2016–2018 NASS, these emergent ambulatory surgeries were undercounted because the NASS was sampled from the SASD without these types of ambulatory surgeries. The procedures most impacted by this issue included appendectomy and removal of ectopic pregnancy (each undercounted by more than 50%) and cholecystectomy (undercounted by approximately 10%).
• The hospital-owned facility universe for the NASS changed between data years 2018 and 2019. First, the universe was expanded to include specialty hospitals such as surgical, cancer, heart, and orthopedic facilities owned by community hospitals that performed in-scope ambulatory surgeries. This resulted in volume increases in certain surgeries commonly performed in these types of facilities (e.g., hip and knee procedures, eye procedures, mastectomies). Second, the universe was limited to hospitals included in the AHA Annual Survey that reported performing outpatient surgeries. In prior years, the CMS POS data were also used to augment this information.
• Because of the change in the participating states in the 2023 NASS, changes were made to the NASS data elements. See What's New In the 2023 AHRQ HCUP NASS?

Refer to the Introduction to the NASS, for 2023 (PDF) and prior years for a summary of CCS-Services and Procedures category totals in the 2016-2023 NASS and contributing reasons for large changes over time. For the subset of CCS-Services and Procedures categories affected by NASS design changes, trend analyses based on CCS-Services and Procedure category are not recommended.

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NASS releases for data years 2016 through 2023 are available for purchase online through the AHRQ HCUP Central Distributor Online Reporting System (CDORS).

All HCUP data users, including data purchasers and collaborators, must complete the online AHRQ HCUP Data Use Agreement Training Tool, and must read and sign the AHRQ HCUP Data Use Agreement for Nationwide Databases (PDF file, 260 KB; HTML).

Questions regarding the purchase or re-use of the data can be directed to the AHRQ HCUP Central Distributor:

E-mail: HCUP-RequestData@ahrq.gov

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The NASS data set is extremely large. The data are distributed as comma-separated value (CSV) files delivered via secure digital download from the AHRQ HCUP Central Distributor Online Reporting System (CDORS). The files are compressed and encrypted with 7-Zip©. To load and analyze the NASS data on a computer, users will need the following:

• The password provided by the HCUP Central Distributor
• A hard drive with at least 85 GB of space available
• A third-party zip utility such as 7-Zip©, ZIP Reader, Secure ZIP®, WinZip®, or Stuffit Expander®. (The built-in utilities in Windows® and Macintosh® cannot decompress or decrypt these zip files and will produce an error message warning of incorrect password and/or file or folder errors.)
• SAS®, SPSS®, Stata® or similar analysis software

The data set includes weights for producing national estimates. NASS documentation and tools, including programs for loading the CSV file into SAS, SPSS, or Stata, are also available on the NASS Database Documentation page.

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