Overview of the Nationwide Ambulatory Surgery Sample (NASS)

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 largest all-payer ambulatory surgery database in the United States, yielding national estimates of major ambulatory surgery encounters performed in hospital-owned facilities. Major ambulatory surgeries are defined as selected major therapeutic procedures that 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). The NASS is limited to encounters with at least one in-scope major ambulatory surgery on the record, performed at hospital-owned facilities. Procedures intended primarily for diagnostic purposes are not considered in-scope. Unweighted, the NASS contains approximately 9.0 million ambulatory surgery encounters each year and approximately 11.8 million ambulatory surgery procedures. Weighted, it estimates approximately 11.9 million ambulatory surgery encounters and 15.7 million ambulatory surgery procedures. Developed through a Federal-State-Industry partnership sponsored by the Agency for Healthcare Research and Quality, HCUP data inform decisionmaking at the national, State, and community levels.


  • Several refinements were made to the NASS sampling design in 2019:

    • The 2019 NASS is based on data selected from both the HCUP State Ambulatory Surgery and Services Databases (SASD) and the State Emergency Department Databases (SEDD) in order to capture both planned and emergent major ambulatory surgeries.
    • The hospital-owned facility universe for the 2019 NASS was expanded beyond general acute care and children's service types. Consistent with all other HCUP nationwide database, the 2019 NASS now includes specialty hospitals such as surgical, cancer, heart, and orthopedic facilities owned by community hospitals that performed in-scope major ambulatory surgeries.
    • Consistent with all other HCUP nationwide databases, the hospital-owned facility universe for the 2019 NASS was limited to hospitals included in the American Hospital Association (AHA) Annual Survey of Hospitals that reported performing outpatient surgeries. In prior years, the Centers for Medicare and Medicaid Services (CMS) Provider of Services (POS) data were used to augment the information on hospital-owned facilities.
    • The list of in-scope procedures was expanded to include heart value procedures, destruction of lesion or retina and choroid, excision of skin lesion, suture of skin and subcutaneous tissue, and other operations on ovary. Two in-scope procedures from the 2018 NASS were excluded: varicose vein stripping and other diagnostic procedures of female organs.
    • These changes will cause some discontinuity in estimates from before and after data year 2019.

  • The 2019 NASS includes several new data elements:

    • Patient race/ethnicity
    • Comorbidity measures identified by the AHRQ Elixhauser Comorbidity Software Refined for ICD-10-CM diagnosis codes
    • Indicators that one or multiple injury-related ICD-10-CM diagnoses are reported on an encounter record were added to the Encounter File

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Sampled from the State Ambulatory Surgery and Services Databases (SASD) and the State Emergency Department Databases (SEDD), HCUP's NASS can be used to create national estimates of major ambulatory surgery encounters performed in hospital-owned facilities. Major ambulatory surgeries are defined as selected major invasive 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).

Key features of the 2019 NASS include:
  • Data from 2,958 hospital-owned facilities located in 34 States and the District of Columbia, approximating a 68-percent stratified sample of U.S. hospital-owned facilities performing selected ambulatory surgeries
  • Data on clinical procedures and diagnoses, disposition of the patient, expected source of payment, and total charges, as well as geographic, hospital-owned facility, and patient characteristics
  • A focus on encounters with at least one "in-scope" ambulatory surgery: an invasive, therapeutic procedure with relatively high procedure volume, a substantial share of procedures in the hospital outpatient setting, and reliable reporting from hospital-owned facilities
  • A supplemental file that provides information on out-of-scope procedures performed during these encounters

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The NASS is delivered as a set of related files.
  • The Hospital File lists provides hospital-level attributes (e.g., teaching status, bed size) for hospital-owned facilities in the NASS as well as the encounter weight and sample stratum information. This hospital-level file contains one observation for each hospital included in the NASS (2,958 hospital-specific records in 2019).
  • The Encounter File links to the Hospital File and contains information on the ambulatory surgery encounter, including patient age, expected source of payment, diagnoses, in-scope and other procedures, charges, and disposition of the patient. This encounter-level file contains 100 percent of ambulatory surgery encounters containing a major ambulatory surgery from the sample of hospitals (9.0 million encounters in 2019).
  • The Supplemental File contains entries for out-of-scope procedures (approximately 6.4 million records in 2019) that were performed during encounters recorded in the Encounter File, with a key linking to the Encounter File.
  • The Diagnosis and Procedure Groups File (available beginning in data year 2018) contains information about ICD-10-CM diagnosis groups for all diagnoses associated with encounters recorded in the Encounter File, with a key linking to the Encounter File.
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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 is composed of more than 100 clinical and nonclinical variables for each encounter. These include:
  • Current Procedural Terminology (CPT®) procedure codes for encounters with at least one major 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
  • Disposition of the patient
  • Hospital characteristics (e.g., region, ownership, teaching status)
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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.
  • Earlier years of the NASS (2016-2018) undercount certain emergent surgeries. Prior to data year 2019, the NASS sample was limited to SASD encounters that involved surgeries defined as "narrow" by the HCUP Surgery Flag Software for Services and Procedures. Subsequent analyses revealed additional encounters involving "narrow" or major surgeries that were started in the emergency department and appeared in the State Emergency Department Databases (SEDD) but not in the SASD. As a result, these surgeries are undercounted in the 2016-2018 NASS. The procedures most impacted by this issue include appendectomy and removal of ectopic pregnancy (each undercounted by more than 50%) and cholecystectomy (undercounted by approximately 10%). In subsequent data years, these emergent in-scope surgeries are captured in the NASS.
  • 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 major 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.

Refer to the Introduction to the NASS document on the NASS Database Documentation page on HCUP-US for a summary of CCS-Services and Procedures category totals in the 2016-2019 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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As a uniform, national database, the NASS promotes wide-ranging studies of health care services 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: Return to Contents
 

The NASS release for data years 2016-2019 are available for purchase through the HCUP Central Distributor.

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

Questions regarding purchasing databases can be directed to the HCUP Central Distributor:

E-mail: hcup@ahrq.gov
Telephone: (866) 290-4287 (toll free)
Fax: (805) 979-3787

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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 Online HCUP Central Distributor. The files are compressed and encrypted with SecureZIP® from PKWARE.

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 50 to 100 GB of space available
  • A third-party zip utility such as ZIP Reader, Secure ZIP®, WinZip®, or Stuffit Expander®
  • 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.

Please note the following based on the software you plan to use:
  • In total, the CSV version of the NASS is about 14.2 GB.
  • The NASS files loaded into SAS are about 6.3 GB. Most SAS data steps will require twice the storage of the file, so that the input and output files can coexist. The largest use of space typically occurs during a sort, which requires work space approximately three times the size of the file. Thus, the NASS files would require approximately 15 GB of available workspace to perform a sort.
  • The NASS files loaded into SPSS are under 8 GB.
With a file this size and without careful planning, space could easily become a problem in a multi-step program with the NASS. It is not unusual to have several versions of a file marking different steps while preparing it for analysis and more versions for the actual analyses; therefore, users should be aware that the amount of space required can escalate rapidly.

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Internet Citation: NASS Overview. Healthcare Cost and Utilization Project (HCUP). October 2021. Agency for Healthcare Research and Quality, Rockville, MD. www.hcup-us.ahrq.gov/nassoverview.jsp.
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Last modified 10/19/21