Overview of the Nationwide Readmissions Database (NRD)

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 readmission rates 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 NRD contains data from approximately 18 million discharges each year. Weighted, it estimates roughly 36 million discharges. 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.


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. The file structure is similar to the file structure of the NRD in data years prior to 2015.


On October 1, 2015, hospital administrative data began using ICD-10-CM/PCS, so the first nine months of 2015 contain ICD-9-CM codes and the last three months contain ICD-10-CM/PCS codes. Data elements and data structure for the 2015 NRD have changed. Trends based on diagnoses or procedures will be affected.

Data elements derived from AHRQ software tools are not available for ICD-10-CM/PCS data on the NRD.
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, 2010-2017 (PDF file, 1.1 MB; HTML).

  • Beginning with the 2017 NRD, separate external cause code data elements are discontinued (formerly HCUP I10_ECAUSEn data elements). External cause codes are now at the end of the ICD-10-CM diagnosis array.
  • The length of the diagnosis array has increased from 35 to 40 codes. Also, the length of the ICD-10-PCS procedure array has increased from 15 to 25 codes.
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The NRD is drawn from the HCUP State Inpatient Databases (SID) and can be used to create estimates of national readmission rates for all patients, regardless of the expected payer for the hospital. The 2017 NRD was constructed from 28 States 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.

Key features of the 2017 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 28 geographically dispersed States, accounting for 60.0 percent of the total U.S. resident population and 58.2 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 is left to the analyst using the NRD.
  • Outcomes of interest include national readmission rates, reasons for returning to the hospital for care, and the hospital costs for discharges with and without readmissions.
  • The NRD is designed to support national readmission analyses and cannot be used for regional, State-, or hospital-specific analyses.
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Beginning with data year 2016, the NRD is a calendar year file that includes diagnoses and procedures coded using only ICD-10-CM/PCS. The file structure is similar to the file structure of the NRD prior to 2015.

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 changes to the HCUP databases for ICD-10-CM/PCS and use of data across the two coding systems may be found on the HCUP-US website under ICD-10-CM/PCS Resources.
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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, 2015
  • 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)
Data elements derived from AHRQ software tools which are based on ICD-10-CM/PCS diagnosis and/or procedure codes are not available on the NRD starting in quarter 4 of 2015. For users interested in applying the AHRQ software tools to the ICD-10-CM/PCS-coded data, the AHRQ software tools are available for download on the HCUP Tools & Software section of the HCUP User Support (HCUP-US) website. The Tools Loading tutorial is available to users interested in applying the AHRQ software tools at www.hcup-us.ahrq.gov/tech_assist/tutorials.jsp.
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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: Return to Contents

The NRD for data years 2010-2017 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, 99 KB; HTML).

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

E-mail: HCUPDistributor@AHRQ.gov
Telephone: (866) 556-4287 (toll free)
Fax: (866) 792-5313 (toll free)

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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 SecureZIP® from PKWARE. To load and analyze the NRD data on a computer, you 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 2017 NRD are 6.2 GB.
  • The NRD files loaded into SAS are about 12 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 NRD files would require approximately 36 GB of available workspace to perform a sort.
  • The NRD files loaded into SPSS are under 16 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 problem in a multi-step program with the NRD. 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: NRD Overview. Healthcare Cost and Utilization Project (HCUP). December 2019. Agency for Healthcare Research and Quality, Rockville, MD. www.hcup-us.ahrq.gov/nrdoverview.jsp?utm_source=AHRQ&utm_campaign=AHRQ_IHIFORUM_2016&utm_medium=TWITTER&utm_term=PROJECT_ECHO&utm_content=4.
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Last modified 12/12/19