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HCUPnet Methodology

The following notes describe special methods used in HCUPnet. For more information, refer to HCUPnet Definitions Glossary.

Table of Contents

Types of hospitals/discharges included in HCUPnet

HCUPnet is based on data from community hospitals in the Healthcare Cost and Utilization Project (HCUP). Community hospitals are defined as short-term, non-federal, general and other hospitals, excluding hospital units of other institutions (e.g., prisons). HCUP data include OB-GYN, ENT, orthopedic, cancer, pediatric, public, and academic medical hospitals. The data exclude long term care, rehabilitation, psychiatric, and alcoholism and chemical dependency hospitals, but these types of discharges are included if they are from community hospitals.

Identifying records with ED visits

Identification of ED visits with admission to the same hospital was based on the following information:

  • A positive charge for emergency department services,
  • CPT® codes 99281-99285,
  • Revenue codes 450-459, or
  • Admission Source or Point of Origin indicating that ED services were received.


Identifying records with ambulatory surgery and comparisons with inpatient surgery

The database used for ambulatory surgery statistics is a combination of all State Ambulatory Surgery and Services Databases (SASD) from HCUP. Statistics on inpatient surgeries are based on the corresponding State Inpatient Databases (SID) for the same states. Number of databases will vary by year based on the availability of data in HCUP.

Ambulatory surgery facilities included all community hospital-affiliated surgery centers. Inpatient facilities included all short-term, acute-care community hospitals. Excluded were rehabilitation hospitals, long-term acute care hospitals, psychiatric hospitals, and substance abuse facilities.

Ambulatory surgeries were selected based on the type of procedures performed. All statistics are limited to invasive surgery commonly performed for therapeutic purposes (i.e., to treat disease or injury).

Some State Ambulatory Surgery and Services Databases use ICD-9-CM codes for procedures, some use CPT® codes, and some use both. CPT® refers to the Common Procedural Terminology, a system used to reimburse for professional services. The Clinical Classification Software (CCS) was developed for use with ICD-9-CM codes, but a version is also maintained for CPT® codes -- CCS for Services and Procedures.

When a state database includes both ICD-9-CM and CPT® procedure codes, the coding system that is predominant (i.e., available for the larger number of records) is used to identify the first-listed procedure. For example, if CPT® codes are predominant in the data, the first-listed CPT® code is used as the first-listed procedure. All remaining codes -- both ICD-9-CM and CPT® -- are considered secondary procedures and are included in all listed procedures.

Community (county) level statistics

HCUPnet provides county- and regional-level statistical tables and maps for single years and three-year aggregate year ranges starting in data year 2011 for those States that have agreed to include their county-level data in HCUPnet. Users can obtain information on volume, rates, and costs for all inpatient discharges in the county or region and by selected diagnosis and procedure categories.

Information can be subdivided further by demographic characteristics such as sex, age group, race/ethnicity, and payer type, when possible. State-level and national benchmarks are also presented for comparison.

For further details, see “Methods - Calculating Community-Level Statistics for HCUPnet” PDF.

Clinical Classifications Software (CCS) and Clinical Classifications Software Refined (CCSR)

CCSR for International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM)

  • The Clinical Classifications Software Refined (CCSR) for ICD-10-CM diagnoses, used beginning in 2016, aggregates more than 70,000 ICD-10-CM diagnosis codes into over 530 clinically meaningful categories. The categories are organized across 21 body systems, which generally follow the structure of the ICD-10-CM diagnosis chapters:
    • Certain Infectious and Parasitic Diseases (INF)
    • Neoplasms (NEO)
    • Diseases of the Blood and Blood Forming Organs and Certain Disorders Involving the Immune Mechanism (BLD)
    • Endocrine, Nutritional and Metabolic Diseases (END)
    • Mental, Behavioral and Neurodevelopmental Disorders (MBD)
    • Diseases of the Nervous System (NVS)
    • Diseases of the Eye and Adnexa (EYE)
    • Diseases of the Ear and Mastoid Process (EAR)
    • Diseases of the Circulatory System (CIR)
    • Diseases of the Respiratory System (RSP)
    • Diseases of the Digestive System (DIG)
    • Diseases of the Skin and Subcutaneous Tissue (SKN)
    • Diseases of the Musculoskeletal System and Connective Tissue (MUS)
    • Diseases of the Genitourinary System (GEN)
    • Pregnancy, Childbirth and the Puerperium (PRG)
    • Certain Conditions Originating in the Perinatal Period (PNL)
    • Congenital Malformations, Deformations and Chromosomal Abnormalities (MAL)
    • Symptoms, Signs and Abnormal Clinical and Laboratory Findings, Not Elsewhere Classified (SYM)
    • Injury, Poisoning and Certain Other Consequences of External Causes (INJ)
    • External Causes of Morbidity (EXT)
    • Factors Influencing Health Status and Contact with Health Services (FAC)
    • Unacceptable principal diagnosis (inpatient data) or first-listed diagnosis (outpatient data) (XXX)


  • ICD-10-CM diagnosis codes are not mutually exclusive and individual ICD-10-CM diagnosis codes sometimes document (1) multiple conditions or (2) a condition and a common symptom or manifestation. Assigning only one CCSR category for these codes requires prioritizing the assignment. HCUPnet uses the default CCSR for the principal diagnosis for inpatient data and the default CCSR for the first-listed diagnosis for outpatient data. The all-listed results presented on HCUPnet are based on the non-mutually exclusive, cross-listed CCSR groupings.
  • Please visit the CCSR for ICD-10-CM Diagnosis website for more information:



CCSR for International Classification of Diseases, Tenth Revision, Procedure Coding System (ICD-10-PCS)

  • The Clinical Classifications Software Refined (CCSR) for ICD-10-PCS procedures, used beginning in 2016, aggregates more than 80,000 ICD-10-PCS procedure codes into over 320 clinically meaningful categories. The categories are organized across 31 clinical domains. Each ICD-10-PCS code is assigned to a mutually exclusive category. HCUPnet uses the CCSR for the first-listed ICD-10-PCS code when “principal” is selected in the query tool.
  • The following acronyms are used within the category descriptions for the CCSR for ICD-10-PCS
    • CABG, Coronary artery bypass graft
    • CNS, Central nervous system
    • COVID-19, Coronavirus Disease 2019
    • EEG, Electroencephalogram
    • ENT, Ear, nose, and throat
    • ERCP, Endoscopic retrograde cholangiopancreatography
    • GI, Gastrointestinal
    • ICP, Intracranial pressure
    • NEC, Not elsewhere classified
    • PCI, Percutaneous coronary intervention
    • Rh, Rhesus factor
  • Please visit the CCSR for ICD-10-PCS Procedures website for more information:


Clinical Classifications Software (CCS) for ICD-9-CM

  • The Clinical Classifications Software (CCS) for ICD-9-CM, used prior to 2016, is a diagnosis and procedure categorization scheme that aggregates over 14,000 diagnosis codes and 3,900 procedure codes into a smaller number of clinically meaningful categories. CCS is based on the International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM), a uniform and standardized coding system.
  • Please visit the CCS for ICD-9-CM Diagnosis and Procedures website for more information:


Note, CCS’s for ICD-9-CM/PCS codes do not easily map onto CCSR’s for ICD-10-CM/PCS codes and thus limit the ability to trend analysis across the transition from ICD-9 to ICD-10 based data.

Diagnosis Related Groups (DRGs) and Major Diagnostic Categories (MDCs)

The Prospective Payment System (PPS) uses approximately 500 DRGs as the basis for payment to hospitals. The "CC" suffix in the DRG labels refers to "complications and comorbidities."

DRGs change annually; a new version is implemented each October 1. As a result, DRG information for the last quarter of each year differs from the DRGs in the first three quarters, but in most years this is a minor change. With the introduction of DRG version 25 in October 2007, the DRG system underwent a major change with the addition and redefinition of numerous DRGs. To avoid this shift in DRG information in HCUPnet for 2007 data, DRG version 24 was retained throughout calendar year 2007 and DRG version 25 was applied beginning with calendar year 2008 data.

Since the 4th quarter of 2008, DRGs require that certain conditions ("healthcare associated conditions" or HACs) have accompanying documentation on whether the conditions were present on admission (POA) or originated during the stay. When POA information for these HACs is missing, cases were assigned to a DRG and MDC indicating the cases were "ungroupable," thus information on the specific condition or body system is lost. Because DRG and MDC information is used in HCUPnet to provide insight into what conditions are being treated in hospitals, NIS data on DRGs and MDCs were created without consideration of whether POA information was available beginning with 2009 data; however, SID data are presented using the original DRG and MDC which requires POA information.

ICD-9 General Conditions (prior to 2016)

HCUPnet categorizes adult inpatient discharges into six types in the following hierarchical order: maternal, neonatal, mental health/substance use, injury, surgical, and medical. Detailed definitions of these general conditions are below. For 2012-2015, HCUPnet allows queries by general conditions. Some general conditions are also available as query options under Focus on Subgroups of Interest.

Users’ note about 2015 data: Due to the transition from ICD-9-CM to ICD-10-CM in October 2015, General Conditions in 2015 can be delineated only in quarters 1 through 3. The number and rate of discharges are suppressed in most queries. However, some queries, such as those selecting an option under Focus on Subgroups of Interest, will produce the number of discharges during quarters 1 through 3, and the rate of discharges during quarters 1 through 3 per 100,000 persons in the total U.S. population (including the entire estimated annual census population). Users are advised to interpret such 2015 results with extreme caution.

Maternal hospitalizations are identified using the Major Diagnostic Category (MDC) 14 Pregnancy, Childbirth and the Puerperium.

Neonatal hospitalizations are identified using the Major Diagnostic Category (MDC) 15 Newborns andNeonates with Conditions Originating in the Perinatal Period.

  • The Focus on Subgroup of Interest category, Non-neonatal, non-maternal discharges are those not categorized as either Maternal or Neonatal hospitalizations.


Mental Health/Substance Use are identified based on the following:

  • Mental Health/Substance Use hospitalizations, beginning with ICD-10-CM, are identified using the Major Diagnostic Category (MDC) 19 Mental Diseases and Disorders and 20 Alcohol/Drug Use and Alcohol/Drug Induced Organic Mental Disorders.

    For ICD-9-CM queries, the Clinical Classifications Software (CCS) for ICD-9-CM was used to identify these hospitalizations, and defined as a principal diagnosis CCS code in any one of the following:

  • From 2007 through 2015
    • 650: Adjustment disorders
    • 651: Anxiety disorders
    • 652: Attention-deficit, conduct, and disruptive behavior disorders
    • 653: Delirium, dementia, and amnestic and other cognitive disorders
    • 654: Developmental disorders
    • 655: Disorders usually diagnoses in infancy, childhood, or adolescence
    • 656: Impulse control disorders, NEC
    • 657: Mood disorders
    • 658: Personality disorders
    • 659: Schizophrenia and other psychotic disorders
    • 660: Alcohol-related disorders
    • 661: Substance-related disorders
    • 662: Suicide and intentional self-inflicted injury
    • 663: Screening and history of mental health and substance abuse codes
    • 670: Miscellaneous disorders
  • From 2003 through 2006
    • 65: Mental retardation
    • 66: Alcohol-related mental disorders
    • 67: Substance-related mental disorders
    • 68: Senility and organic mental disorders
    • 69: Affective disorders
    • 70: Schizophrenia and related disorders
    • 71: Other psychoses
    • 72: Anxiety; somatoform; dissociative; and personality disorders
    • 73: Preadult disorders
    • 74: Other mental conditions
    • 75: Personal history of mental disorder; mental and behavioral problems; observation and screening for mental condition


Injury discharges are identified by a principal diagnosis in the following ranges of ICD-10-CM and ICD-9-CM codes:

ICD-9-CM Codes Prior to October 1, 2015

  • 800-909.2, 909.4, 909.9: Fractures; dislocations; sprains and strains; intracranial injury; internal injury of thorax, abdomen, and pelvis; open wound of the head, neck, trunk, upper limb, and lower limb; injury to blood vessels; late effects of injury, poisoning, toxic effects, and other external causes, excluding those of complications of surgical and medical care and drugs; medicinal or biological substances.
  • 910-994.9: Superficial injury; contusion; crushing injury; effects of foreign body entering through orifice; burns; injury to nerves and spinal cord; traumatic complications and unspecified injuries; poisoning and toxic effects of substances; other and unspecified effects of external causes.
  • 995.5-995.59: Child maltreatment syndrome.
  • 995.80-995.85: Adult maltreatment, unspecified; adult physical abuse; adult emotional/psychological abuse; adult sexual abuse; adult neglect (nutritional); other adult abuse and neglect.

The above definition of injury includes five ICD-9-CM diagnosis codes that are also included under two CCS diagnosis categories used for the definition of the mental health/substance use hospitalization type:

  • CCS = 660 (Alcohol-related disorders): diagnosis 980.0 (toxic effect of ethyl alcohol)
  • CCS = 661 (Substance-related disorders): diagnoses 965.00 (poisoning by opium), 965.01 (poisoning by heroin), 965.02 (poisoning by methadone), 965.09 (poisoning by other opiate).

Because of the hierarchical ordering used to assign discharges to hospitalization type, discharges with one of these five principal ICD-9-CM diagnosis codes are assigned to the mental health/substance use hospitalization type and not the injury hospitalization type.

It should be noted that ICD-9-CM and ICD-10-CM diagnoses codes related to complications of surgical or medical care, or any adverse events or anaphylactic shock resulting from medication, anesthesia, or food are not used in the definition of the injury hospitalization type.


Surgical discharges are identified by a surgical diagnosis-related group (DRG). The DRG grouper first assigns the discharge to a major diagnostic category (MDC) based on the principal diagnosis. For each MDC, there is a list of procedure codes that qualify as operating room procedures. If the discharge involves an operating room procedure, it is assigned to one of the surgical DRGs within the MDC category; otherwise it is assigned to a medical DRG. If the DRG indicates the information on the record is ungroupable (i.e., not identifiable as medical or surgical), then the discharge is assumed to be medical. This rarely occurs (less than 0.1 percent of total discharges).

Medical discharges are identified by a medical DRG. The DRG grouper first assigns the discharge to an MDC based on the principal diagnosis. For each MDC there is a list of procedure codes that qualify as operating room procedures. If the discharge involves an operating room procedure, it is assigned to one of the surgical DRGs within the MDC category; otherwise it is assigned to a medical DRG. If the DRG indicates the information on the record is ungroupable (i.e., not identifiable as medical or surgical), then the discharge is assumed to be medical. This rarely occurs (less than 0.1 percent of total discharges).

ICD-10 General Conditions (beginning in 2016)

All discharges are categorized into five hospitalization types (i.e., service lines) in the following hierarchical order: maternal/neonatal, mental health/substance abuse, injury, surgical, and medical. The criteria for identifying the hospitalization types varies across data years.

Beginning in data year 2019, the five hospitalization types are defined as follows:

Maternal and neonatal discharges are defined using the following Major Diagnostic Categories (MDC):

  • MDC 14 Pregnancy, Childbirth and Puerperium
  • MDC 15 Newborn and Other Neonates (Perinatal Period)


Mental health/substance abuse discharges are defined using the following MDCs:

  • MDC 19 Mental Diseases and Disorders
  • MDC 20 Alcohol/Drug Use or Induced Mental Disorders


Injury discharges are identified using the Clinical Classification Software Refined (CCSR) categories for the principal ICD-10-CM diagnosis:

  • INJ001 - INJ027
  • INJ032


Surgical discharges are identified by a surgical DRG. The DRG grouper first assigns the discharge to a Major Diagnostic Category (MDC) based on the principal diagnosis. For each MDC, there is a list of procedure codes that qualify as operating room procedures. If the discharge involves an operating room procedure, it is assigned to one of the surgical DRGs within the MDC category.

All other discharges that are not assigned to a surgical DRG are identified as a medical discharge. If the DRG indicated the information on the record was ungroupable (i.e., not identifiable as medical or surgical), then the discharge was assumed to be medical. This rarely occurred (less than 0.1 percent of total discharges).

ICD-10-CM Diagnosis and ICD-10-PCS Procedure Codes

The option to query individual ICD-10-CM/PCS codes for utilization and outcome statistics has been disabled because the results may not be indicative of the intended clinical/surgical concept. Individual ICD-10-CM diagnoses or ICD-10-PCS procedures often need to be reported in combination to define a clinical/surgical concept, because of the specificity of the individual codes. The ICD-10-CM Coding Guidelines provide a number of examples. HCUPnet provides the ability to query clinical conditions and surgeries by Clinical Classification Software Refined (CCSR), which aggregates ICD-10-CM/PCS into clinically meaningful categories. As an alternative, 'Frequencies by Diagnosis and Procedure Codes' (individually and by the CCSR categories) are available in the Documentation section of each nationwide database at the following links:



Combining ICD-9-CM/PCS All-listed Diagnosis or Procedure Codes

When you query all-listed diagnoses for multiple ICD-9-CM/PCS codes and request statistics on all codes combined, individual visits may be counted more than once if multiple ICD-9-CM/PCS codes appear on a visit record. This means the unit of analysis is unique ICD-9-CM/PCS codes rather than visits.

Highest severity of illness

Cases with the highest severity of illness are discharges with a score of 3 or 4 on the APR-DRG severity of illness scale. The four severity of illness subclasses are numbered sequentially from 1 to 4 indicating minor, moderate, major, or extreme severity of illness. The determination of severity of illness is disease-specific. Thus, the significance attributed to complicating or comorbid conditions is dependent on the underlying problem. For example, certain types of infections are considered a more significant problem in a patient who is immunosuppressed than in a patient with a fractured arm. In APR-DRGs, high severity of illness is primarily determined by the interaction of multiple diseases. Patients with multiple comorbid conditions involving multiple organ systems represent difficult-to-treat patients who tend to have poor outcomes. The assignment of a patient to a severity of illness subclass takes into consideration not only the level of the secondary diagnoses but also the interaction among secondary diagnoses, age, principal diagnosis, and the presence of certain OR procedures and non-OR procedures.

Conversion of charges to costs

HCUP data include information on facility charges, that is, what hospitals bill for services. In order to provide an estimate of the costs of providing those services (the actual costs of production), total charges were converted to costs using cost-to-charge ratios based on hospital accounting reports from the Centers for Medicare and Medicaid Services (CMS). In general, costs are less than charges. For each hospital, a hospital-wide cost-to-charge ratio is used because detailed charges are not available across all HCUP States.

There is a time lag in the information used to convert charge data to costs, thus cost data may appear in HCUPnet later than charge data.

When a cost-to-charge ratio was missing for a hospital, the average ratio for the hospital's stratum (as defined by bedsize, teaching status, location, ownership, and region) was used.

Users calculating costs with raw HCUP data and the public use version of the cost-to-charge ratio file will notice slight discrepancies with the cost values provided on HCUPnet. The reason for this difference is that the public use version of the CCR file masks identities of some hospitals by using a mean cost-to-charge ratio, while HCUPnet uses actual cost-to-charge ratios for all hospitals.

Costs are not reported for patient's urban/rural location because no cost of living adjustment is made.

Costs for stays with a high proportion of ancillary charges (e.g., major surgery) will tend to be overestimated using these cost-to-charge ratios while costs for cases with a high proportion of room and board charges (e.g., tracheostomy, mental illness, premature newborns) will tend to be underestimated.

In HCUP, it is not possible in general to estimate costs for each inpatient stay based on the use of specific services or departments. However, in ten HCUP States that reported detailed charges for each discharge in 2006, the cost for stays in each CCS diagnostic category and in each APR-DRG category was calculated using department-level cost-to-charge ratios from the CMS accounting system. Such cost estimates are more accurate than the cost estimation methods reported in HCUPnet. However, less than one-third of cases had a cost that differed from using hospital-wide cost-to-charge ratios by 10%. Most categories needing an upward adjustment of cost were in labor and delivery, newborns, nervous system, cerebrovascular accident, and mental health problems. Categories needing a downward adjustment in estimated cost were in surgical procedure cases such as cardiovascular surgery and hip or knee replacement. For details on tests and data that can be used to adjust cost for more accuracy, see the HCUP Methods Series, report 2008 #4 (126 KB; PDF Help).

From 2004-2008, mean and aggregate costs for the Northeast region are slightly overestimated relative to other regions and years because hospitals from Pennsylvania were not available for sampling during those years. However, estimates are reliable when comparing groups of cases across hospital types, patient characteristics, or diagnosis and procedure clusters.

Aggregate charges and costs

When a case was missing information on charges (and costs), a value was imputed by taking the mean charges for all discharges of the same DRG with non-missing charges. Fewer than 2% of cases are missing charges in HCUP data. Because of how missing charges are imputed, simple calculation of number of discharges X mean charge (or costs) will not always equal the aggregate charges (costs) shown in HCUPnet.

Admission source

The definition changed in 2007. Not all data sources had adopted the change at that time; therefore, information on source of admission is not available for 2007 and after.

Hospital characteristics

Information on hospital characteristics is taken from American Hospital Association's Annual Survey. Most of these characteristics are the same as those found in the publicly available versions of the HCUP National (Nationwide) Inpatient Sample (NIS), except for hospital ownership/control. In 1998, the NIS underwent a redesign and the categories of hospital ownership/control were redefined for sampling and reporting purposes. In order to provide continuity of statistics, the pre-1998 definition of hospital ownership/control is used for all years of data and all datasets in HCUPnet.


HCUPnet provides information on 7-day and 30-day hospital readmissions. A 7-day readmission is a subsequent hospital admission in the same or a different hospital within 7 days following an original admission (or index stay). A 30-day readmission is a subsequent hospital admission in the same or a different hospital within 30 days following an original admission (or index stay). For 30-day readmissions, the discharge date for the index stay must occur between January and November to allow a 30-day follow-up period for all index stays. The 30-day readmission rate is defined as:

  • the number of stays with at least one subsequent hospital stay within 30 days
  • the total number of hospital stays between January and November

For further details, see "Methods - Calculating Readmissions for HCUPnet" HTML.


"Payer" is the expected payer for the hospital stay, based on the first-listed payer. To make coding uniform across all HCUP data sources, Payer combines detailed categories into more general groups:

  • Medicare includes fee-for-service and managed care Medicare patients;
  • Medicaid includes fee-for-service and managed care Medicaid patients;
  • Private insurance includes Blue Cross, commercial carriers, and private HMOs and PPOs;
  • Uninsured includes an insurance status of "self-pay" and "no charge;"
  • Other includes Worker's Compensation, TRICARE/CHAMPUS, CHAMPVA, Title V, and other government programs.

When more than one payer is listed for a hospital discharge, the first-listed payer is used.

Patients covered by the State Children's Health Insurance Program (CHIP) may be included under Medicaid, private insurance or other insurance, depending on the structure of the state program. Because most state data do not identify CHIP patients specifically, it is not possible to present this information separately in HCUPnet.

Rural/urban location of patient’s residence

For 2003-2006, rural/urban location of patient’s residence is a simplified adaptation of the 2003 version of the Urban Influence Codes (UIC), which divide counties into categories based on size and proximity to population centers:

  • "Large metropolitan" includes metropolitan areas of 1 million people or more.
  • "Small metropolitan" includes metropolitan areas of at least 50,000 but fewer than 1 million people.
  • "Non-metropolitan" includes all micropolitan areas (core-based statistical areas of more than 10,000 people but less than 50,000) and all non-urban (rural) areas.

Beginning with 2007, the rural/urban categorization is a simplified adaptation of the 2006 version of a system developed by the National Center for Health Statistics (NCHS) especially for use in healthcare research. The classification emphasizes urban distinctions and is unique in differentiating between central and fringe counties of large metropolitan areas. Smaller metropolitan counties are subdivided by population. Non-metropolitan counties are divided simply into micropolitan and non-core categories.

The NCHS county classifications are based on the Office of Management and Budget (OMB) metropolitan/micropolitan assignments as updated through the 2005 revisions (OMB Bulletin No. 06-01). These assignments were refined using information from the Rural-Urban Continuum Code (RUCC) and Urban Influence Code (UIC) of the Economic Research Service of the U.S. Department of Agriculture and county characteristics from the 2000 Census and 2004 Census Bureau population estimates.

  • "Large central" refers to counties of metro areas of 1 million or more population, and can be considered cores of large cities.
  • "Large fringe" refers to counties of metro areas of 1 million or more population and can be considered suburbs of large cities.
  • "Medium and small metro" refers to counties in metro areas with less than 1 million population but with at least 50,000.
  • "Micropolitan and noncore" refers to rural counties with less than 50,000 population.

Safety-net status

Using the State Inpatient Databases (SID), the percentage of Medicaid and uninsured discharges was calculated out of all discharges at each hospital. Hospitals in the National Inpatient Sample (NIS) were used. These include U.S. community hospitals, excluding rehabilitation and long-term acute care hospitals. Using hospitals from all states, safety-net hospitals were defined as those in which the percentage of Medicaid and uninsured discharges fell in the top quartile for the nation as a whole (that is, not within each state).

NIS results prior to 2012 were recalculated

Beginning with 2012, the National Inpatient Sample (NIS) was redesigned to optimize national estimates. As a result, the nationwide statistics for 2011 and earlier years on HCUPnet were regenerated using new trend weights in order to permit longitudinal analysis. The regenerated data were posted to HCUPnet on 7/2/2014.

The statistics for data years 2011 and earlier currently on HCUPnet will differ slightly from statistics obtained prior to 7/2/2014. They also differ from statistics generated using your own copy of the NIS and the original discharge weights (DISCWT). However, at the bottom of each results table, you will find a link to the same query using the original discharge weights.

More information about the NIS redesign and trend weights (TRENDWT) can be accessed in the Overview of the NIS, on the HCUP User Support Web site (

Suppressed cases

For nationwide data, statistics based on estimates with a relative standard error (standard error / weighted estimate) greater than 0.30 or with standard error = 0 are not reliable. These statistics are suppressed and are designated with an asterisk (*).

The estimates of standard errors in HCUPnet were calculated using SUDAAN software. These estimates may differ slightly if other software packages are used to calculate variances.

Values based on 10 or fewer discharges or fewer than 2 hospitals in the State statistics (SID) are suppressed to protect confidentiality of patients and are designated with an asterisk (*).

Excluded cases

Some cases with very high values of length of stay or charges were not included in the analyses. For data 1997 and earlier for the National (Nationwide) Inpatient Sample, cases with length of stay longer than 365 days or total charges higher than $5 million were dropped from HCUPnet analyses.

For data 1998 and later, for all databases (nationwide and state), values of length of stay longer than 365 days or total charges higher than $10 million values were set to missing during HCUP data processing. Thus, no cases were excluded after 1997. When length of stay was missing, total charges was also set to missing.

Matching your calculations to results from HCUPnet

If you are using HCUPnet to verify your own calculations from the National (Nationwide) Inpatient Sample (NIS), you may notice small discrepancies in some estimates. HCUPnet uses data that are stored as SAS files. The NIS files that you purchased were sent to you as ASCII files. Weights (for making national estimates) in the ASCII files are truncated at the fourth decimal place, thus some resulting estimates will be slightly different from those in HCUPnet; however, the differences should be very small. For example, differences in total charges will likely disappear if you round estimates to the nearest $100.

SAS and SAS-callable SUDAAN were used to generate the statistics in HCUPNet. Most recently for 2018, 64-bit SAS 9.4 and SUDAAN 11.0.3 were used; however, many older versions of SAS and SUDAAN have been used to generate statistics in HCUPnet. Variances estimated by other statistical packages may differ from SAS and SUDAAN.

Differences between HCUPnet and Fast Stats

In some cases, slightly different definitions were used to present the information in HCUPnet and Fast Stats. Two specific instances of differences are in the definitions of the maternal and neonatal hospitalization types, and in the version of the CCS software that is used to classify diagnoses and procedures.

The first source of differences lies in how maternal and neonatal records are defined. For HCUPnet, the definitions of maternal and neonatal use Major Diagnostic Categories (MDCs) to classify diagnosis codes. For Fast Stats, the definitions of maternal and neonatal rely on the Clinical Classifications Software (CCS). Compared with using MDCs, the CCS approach assigns approximately 0.9 percent fewer cases to "maternal” because a maternal discharge is classified into a mental health CCS or a substance use CCS when the set of diagnosis codes includes a mental health or substance abuse condition along with a maternal condition (e.g., drug dependence in pregnancy). Similarly, compared with the MDC approach, the CCS approach assigns 0.1 percent fewer cases to “neonatal” because a neonatal discharge is classified into a substance use CCS when the set of diagnosis codes includes a drug effect on the fetus or neonatal drug withdrawal. The CCS approach assigns another 0.1 percent fewer cases to ”neonatal” than the MDC approach because neonatal septicemia is assigned to the septicemia CCS rather than a neonatal CCS.

A second source of differences lies in the timing of analysis. Because tables for HCUPnet are generated as soon as each year's database is completed, HCUPnet uses the CCS version provided on each year of the NIS. For NIS 2018, CCSR 2021.2 was used. For Fast Stats, the CCS categories are assigned to the National (Nationwide) Inpatient Sample (NIS) based on the most current CCS software. This affects the numbering scheme, the counts, and the labels for the CCS.

For data years 2016 and later, HCUPnet uses the Clinical Classification Software Refined (CCSR) version that is most current at the time the estimates are produced:


  • NEDS, 2017: v2021.1
  • NIS, 2018: v2021.2


Population-based Rates

Population-based rates are calculated using national population estimates that correspond to the year of HCUP data. The population estimates are taking from the HCUP Methods Report: "Population Denominator Data for Use with the HCUP Databases," which is available from the HCUP Methods Series.

Population-based rates are only available for national estimates and only available for categories of patient or hospital characteristics that are in the HCUP Methods Report—for age group, sex, median income of patient's ZIP code, location of patient's residence, region of the U.S., and census division of the U.S. When the children only (age 0-17) subgroup of interest is selected, rates are only available for age group and sex.

The rate is calculated as number of discharges or visits divided by the population estimate. The standard error of the rate is calculated as the standard error of the number of discharges or visits divided by the population estimate.

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