The Nationwide Readmissions Database (NRD) is part of a family of databases and software tools developed for the Healthcare Cost and Utilization Project (HCUP). The NRD is a unique and powerful database designed to support various types of analyses of national readmissions for all patients, regardless of the expected payer for the hospital stay. This database addresses a large gap in healthcare data - the lack of nationally representative information on hospital readmissions for all ages. Unweighted, the 2021 NRD contains data from approximately 16.8 million discharges. Weighted, it estimates roughly 33.4 million discharges. Developed through a Federal-State-Industry partnership sponsored by the Agency for Healthcare Research and Quality (AHRQ), HCUP data inform decision making at the national, State, and community levels.

THE NRD CONTAINS A FULL YEAR OF ICD-10-CM/PCS CODES BEGINNING WITH DATA YEAR 2016

The NRD includes a full calendar year of data with diagnosis and procedure codes reported using the ICD-10-CM/PCS¹ coding system beginning data year 2016. The file structure is similar to the file structure of the NRD in data years 2014 and prior years.

¹ ICD-9-CM: International Classification of Diseases, Ninth Revision, Clinical Modification. ICD-10-CM/PCS: International Classification of Diseases, Tenth Revision, Clinical Modification/Procedure Coding System.

This page provides an overview of the NRD. For more details, see NRD Database Documentation and the Introduction to the NRD, 2021 (PDF file, 1.7 MB).

Contents:

• What's New in the 2021 NRD?
• About the NRD
• NRD Data Elements
• NRD File Structure
• NRD Areas of Research and HCUP Publications
• Purchase the NRD
• NRD Hardware and Software Requirements

• The overall number of discharges for data year 2021 increased by about 3 percent from 2020. The largest increase was in the second quarter, which had about 18 percent more discharges than the prior year. The overall 30-day all-cause readmission rate remained stable at 13.9 percent.

The NRD is a unique and powerful database designed to support various types of analyses of national readmissions for all patients regardless of the expected payer for the hospital stay. The NRD includes discharges for patients with and without repeat hospital visits in a year and those who have died in the hospital. Repeat stays may or may not be related. The criteria to determine the relationship between hospital admissions are left to the analyst using the NRD. This database addresses a large gap in healthcare data - the lack of nationally representative information on hospital readmissions for all ages.

The NRD is drawn from the HCUP State Inpatient Databases (SID) with reliable, verified patient linkage numbers in the SID that could be used to track the patient across hospitals within a State, while adhering to strict privacy guidelines. There are 30 HCUP Partners that contributed to the 2021 NRD.

Key features of the 2021 NRD include:

• A large sample size, which provides sufficient data for analysis across hospital types and the study of readmissions for relatively uncommon disorders and procedures.
• Discharge data from 30 geographically dispersed States, accounting for 61.2 percent of the total U.S. resident population and 59.6 percent of all U.S. hospitalizations.
• Designed to be flexible to various types of analyses of readmissions in the United States for all types of payers.
• Criteria to determine the relationship between multiple hospital admissions for an individual patient in a calendar year left open to the analyst using the NRD.
• Designed to support national readmission analyses but not for use in regional, State-, or hospital-specific analyses.

The 2021 NRD is an annual, calendar file that includes three discharge-level files and one hospital-level file:

Discharge-level files

• Core File is a single file containing commonly used data elements (e.g., age, expected primary payer, discharge status, ICD-10-CM/PCS codes, total charges).
  • This file is available in all years of the NRD.
• 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 in all years of the NRD.
• Diagnosis and Procedure Groups File is a single file containing additional information on the ICD-10-CM diagnoses and ICD-10-PCS procedures created by the HCUP software tools.
  • For data years 2016-2017, this file was not available in the NRD. Data elements derived from the ICD-10-CM/PCS HCUP software tools are not included in the NRD because they were still in development and testing.
  • Beginning with data year 2018, this file is available in the NRD 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.

Hospital-level files

• Hospital File is a single file containing information on hospital characteristics.
  • This file is available in all years of the NRD.

For 2015 data, because of the transition to ICD-10-CM/PCS on October 1, 2015, data elements related to diagnoses and procedures are included in files that are split into two parts. Nine months of the 2015 data with ICD-9-CM codes (discharges from January 1, 2015 - September 30, 2015) are in one set of files labeled Q1-Q3. Three months of 2015 data with ICD-10-CM/PCS codes (discharges from October 1, 2015 - December 31, 2015) are in a separate set of files labeled Q4. More information about the file structure changes for data year 2015 are included the document, 2015 Healthcare Cost and Utilization Project (HCUP) Nationwide Readmissions Database: Change in Structure and Data Elements Caused by Transition to ICD-10-CM/PCS.

The NRD contains clinical and nonclinical variables that support readmission analyses, with safeguards to protect the privacy of individual patients, physicians, and hospitals. There is no data element identifying whether sequential inpatient stays are related or unrelated. The criteria to determine the relationship between hospital admissions is left to the analyst using the NRD.

The NRD is comprised of more than 100 clinical and nonclinical variables for each hospital stay, including:

• Variables essential to readmission analyses
  • Verified patient linkage number that identifies discharges belonging to the same individual
  • Timing between admissions for a patient
  • Length of inpatient stay in days
  • Identification of transfers, same-day stays, and combined transfer records
  • Identification of the patient as a resident of the State in which he or she received hospital care
• Variables essential to calculating national estimates
  • Discharge weight for generating national estimates
  • Stratum used for weighting
• International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) diagnosis, procedure, and external cause of injury codes prior to October 1, 2015
• International Classification of Diseases, Tenth Revision, Clinical Modification/Procedure Coding System (ICD-10-CM/PCS) diagnosis, procedures, and external cause of morbidity codes beginning October 1, 2015²
• Data elements derived from the HCUP software tools (not available for ICD-10-CM/PCS-coded data in the 2015 Q4-2017 NRD³)
• Patient demographics (e.g., sex, age, median household income quartile, and urban/rural location of the patient's residence)
• Expected payment source (e.g., Medicare, Medicaid, private insurance, self-pay, those billed as 'no charge', and other insurance types)
• Total charges and hospital cost (calculated using the Cost-to-Charge Ratio file)

² Beginning in data year 2016, the NRD includes diagnosis and procedure codes reported using only the ICD-10-CM/PCS coding system. The names of diagnosis- and procedure-related data elements under the ICD-10-CM/PCS coding system begin with the prefix "I10_" to identify the coding scheme.
³ Users interested in applying HCUP software tools to the NRD to produce data elements unavailable in the database files for a certain year 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 HCUP software tools to HCUP or other administrative databases.

As a uniform, multi-State weighted database, the NRD promotes comparative studies of healthcare services and supports healthcare policy and research on a variety of topics, including:

• National readmission rates by diagnosis, procedure, patient demographics, or expected payment source
• Costs associated with readmissions
• Reasons for readmissions
• Impact of health policy changes
• Readmissions by special populations.

The NRD is used in a variety of publications and online tools:

• HCUP Statistical Briefs highlight a variety of health topics.
• Use the HCUP Publications Search Tool to find publications using the NRD.
• Review featured publications on the HCUP Research Spotlights page.
• HCUPnet is an online query system for identifying, tracking, analyzing, and comparing statistics on inpatient and outpatient care based on the HCUP databases.

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The NRD for data years 2010-2021 are available for purchase online 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 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: (866) 551-4587

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

• The password provided by the HCUP Central Distributor
• A hard drive with at least 50 gigabytes (GB) of space available
• A third-party zip utility such as ZIP Reader, 7-Zip© or WinZip®, SecureZIP®, WinZip®, or Stuffit Expander®
• SAS®, SPSS®, Stata® or similar analysis software

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

Please note the following based on the software you plan to use:

• In total, the CSV files for the 2021 NRD are 38 GB.
• The NRD files loaded into SAS are about 10 GB with compression. 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 NRD files would require approximately 30 GB of available workspace to perform a sort procedure.
• The NRD files loaded into SPSS are under estimated 21 GB.
• Because Stata loads the entire file into memory, it may not be possible to load every data element in the NRD Core file into Stata. Stata users will need to maximize memory and use the "_skip" option to select a subset of variables.

With a file this size and without careful planning, space could easily become a multi-step process with the NRD. It is common to produce several versions of a file during data preparation, as well as further multiple versions for analysis. Therefore, the amount of space required could escalate rapidly.

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