Inpatient Stays

The national estimates presented in this section of Fast Stats are from the HCUP National (Nationwide) Inpatient Sample (NIS). The NIS is based on data from community hospitals, which are defined as short-term, non-Federal, general, and other hospitals, excluding hospital units of other institutions (e.g., prisons). The NIS includes obstetrics and gynecology, otolaryngology, orthopedic, cancer, pediatric, public, and academic medical hospitals. Excluded are community hospitals that are also long-term care facilities such as rehabilitation, psychiatric, and alcoholism and chemical dependency hospitals. Beginning in 2012, long-term acute care hospitals (LTACs) are also excluded from the sampling frame. However, if a patient received long-term care, rehabilitation, or treatment for psychiatric or chemical dependency conditions in a community hospital, the discharge record for that stay will be included in the NIS.

The NIS is sampled from the HCUP State Inpatient Databases (SID). Beginning with the 2012 data year, the NIS is a 20 percent sample of discharges from all community hospitals participating in HCUP in that data year. For data years 1988 through 2011, the NIS was a 20 percent sample of community hospitals and included all discharges within sampled hospitals. The national estimates presented in this section of Fast Stats were developed using the NIS Trend Weight Files for consistent estimates across all data years (e.g., LTACs were removed from analysis using trend weights).

Unit of Analysis

The unit of analysis in the NIS is the hospital discharge (i.e., the inpatient stay), not a person or patient. This means that a person who is admitted to the hospital multiple times in one year is counted each time as a separate discharge from the hospital. Counts are summarized by discharge year. There were no exclusions applied to the data (e.g., transfers to another acute care hospital are included as separate hospital stays).

Clinical Coding

On October 1, 2015, the United States transitioned from ICD-9-CM¹ to ICD-10-CM/PCS². The 2015 rates of stays per 100,000 population in this section of HCUP Fast Stats are based on the first three quarters of data with ICD-9-CM codes only (January 1, 2015 to September 30, 2015). The number of inpatient stays by diagnosis in 2015 is not reported because the statistics are not based on full year data. More information on the impact of ICD-10-CM/PCS is available on the HCUP User Support (HCUP-US) web page for ICD-10-CM/PCS Resources.

¹ International Classification of Diseases, Ninth Revision, Clinical Modification
² International Classification of Diseases, Tenth Revision, Clinical Modification/Procedure Coding System

Operating Room Procedures

Operating room (OR) procedures are identified using the Procedure Classes for ICD-9-CM for data years 2015 and earlier and the Procedure Classes Refined for ICD-10-PCS beginning data year 2016. The Procedure Classes tools identify procedures as diagnostic or therapeutic and whether they would be expected to be performed in an operating room. OR procedures are identified using all-listed procedures (principal and secondary) that are available on the discharge record.

The definition of an OR procedure varies over time. With the Procedure Classes for ICD-9-CM, procedure codes are considered to be valid OR procedures based on the definition of a major surgery in the Medicare Severity Diagnosis Related Group (MS-DRG) grouper. The MS-DRG classification scheme relied on physician panels that classified ICD-9-CM procedure codes according to whether the procedure would be performed in a hospital OR in most hospitals. Beginning with the Procedure Classes Refined for ICD-10-PCS v2021.2, ICD-10-PCS procedure codes are determined to be valid OR procedures based on the AHRQ Quality Indicator (QI) software. Changes to either the definition of an OR procedure or the coding system may affect how some procedures are ranked. For example, circumcision is no longer reported as a most common OR procedure beginning data year 2016.

Results are reported using the Clinical Classifications Software (CCS) for ICD-9-CM for data years 2015 and earlier, and the Clinical Classifications Software Refined (CCSR) for ICD-10-PCS procedure codes beginning with data year 2016. Results are reported by the CCS or CCSR category and list the top 10 most common operations for each data year. Counts for procedures are de-duplicated within a discharge record: if a particular CCS or CCSR procedure occurs multiple times during the same hospital stay, it is counted only once. Because there can be multiple procedures reported on a single hospital stay, one discharge record may contribute to the count for more than one operating room procedure. The top 10 ranking is based on the weighted number of stays. Because of the transition from ICD-9-CM to ICD-10-CM/PCS on October 1, 2015, the total number of inpatient stays in 2015 is not reported. The 2015 rate of stays per 100,000 population is based on the first three quarters of 2015 data (Q1-3) only.

Results can be displayed with maternal and neonatal stays included or excluded from the ranking. This option is provided because maternal and neonatal discharges account for nearly a fourth of all hospital discharges in a year and the majority are low complexity, low cost stays. Maternal and neonatal stays are defined differently across data years. For data years 2015 and earlier, the principal diagnosis CCS 176 through 196 are used for maternal and CCS 218 through 224 are used for neonatal. Beginning with data year 2016, Major Diagnostic Category (MDC) 14 (Pregnancy, Childbirth, and the Puerperium) and MDC 15 (Newborns and Other Neonates with Conditions Originating in the Perinatal Period) are used.

Rate of Stays per 100,000

The rate of stays includes the HCUP number of stays in the numerator and the U.S. resident population in the denominator (with a multiplier of 100,000). The denominator is consistently defined with the numerator (i.e., rates for females use HCUP counts and population counts specific to females). Population data are obtained from Claritas, a vendor that produces population estimates and projections based on data from the U.S. Census Bureau. Claritas estimates intercensal annual household and demographic statistics for geographic areas. Rates are not reported by expected payer because currently there is no source for national population insurance estimates that align with HCUP's definition of expected primary payer. Because of the transition from ICD-9-CM to ICD-10-CM/PCS on October 1, 2015, the rate of stays per 100,000 population is based on the first three quarters of 2015 data (Q1-3) only.

Age

Age refers to the age of the patient at admission. Discharges missing age are excluded from results reported by age.

Sex

All nonmale, nonfemale responses are set to missing. Discharges with missing values for sex are excluded from results reported by sex.

Expected Payer

The "expected payer" data element in HCUP databases provides information on the type of payer that the hospital expects to be the source of payment for the hospital bill. Information is reported by the following expected primary payers: Medicare, Medicaid, private insurance, and self-pay/no charge. Patients identified as self-pay/no charge have an expected primary payer of self-pay, no charge, charity, or no expected payment. More information on expected payer coding in HCUP data is available in HCUP Methods Series Reports by Topic "User Guide - An Examination of Expected Payer Coding in HCUP Databases" (multiple documents; updated annually). Discharges missing expected payer are excluded from results reported by expected payer.

Discharges with the following expected primary payers are not reported in Fast Stats reporting for most common operations during inpatient stays: other Federal, State, and local programs; missing; or invalid.

Community-Level Income

Community-level income is based on the median household income of the patient's ZIP Code of residence. Quartiles are defined so that the total U.S. population is evenly distributed across four groups. Over time, the data element in the NIS for community-level income has changed definitions. Starting in data year 2002, the cut-offs for the quartile designation are determined annually using ZIP Code demographic data obtained from Claritas, a vendor that produces population estimates and projections based on data from the U.S. Census Bureau. Claritas estimates intercensal annual household and demographic statistics for geographic areas. The value ranges for the national income quartiles vary by year. Information by community-level income is only reported from 2002 forward because of inconsistent definitions over time in the income-related data elements in the NIS. Income quartile is missing if the patient is homeless or foreign. Discharges missing the income quartile are excluded from results reported by community-level income.