studying injuries using the national hospital discharge survey marni hall, ph.d. hospital care...
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Studying Injuries Using the National Hospital Discharge Survey
Marni Hall, Ph.D.Hospital Care Statistics Branch,
Division of Health Care Statistics
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICESCenters for Disease Control and PreventionNational Center for Health Statistics
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICESCenters for Disease Control and PreventionNational Center for Health Statistics
Outline of this presentation
• Present general information about the design of the National Hospital Discharge Survey (NHDS)
• Discuss decisions that have to be made when designing injury research using NHDS
• Highlight issues particularly related to trend analyses in
Upcoming Chartbook
Trends in Injury Hospitalization, United States, 1979-2001
by Melissa Heinen, Marni Hall, Manon Boudreault, and Lois Fingerhut
NHDS
a national probability sample of short-stay non-federal hospitals - conducted
every year since 1965
provides data on discharges or hospitalizations – not individuals
2002 data - now available
2003 data - available in the winter
NHDS Design
Three stage design
Geographic Units Hospitals Discharges
Data Collection
Automated – 40%
Manual – 60%
Patient Data
• Sex
• Race
• Age
• Expected source of payment
• Discharge status – including deaths
Facility Characteristics
• Geographic region
• Bed size
• Ownership
Medical Data
Coded according to the International Classification of Diseases, 9th Revision,
Clinical Modification (ICD-9-CM)
• Diagnoses
• Surgical and nonsurgical procedures
Additional Variables
• Days of care• Month of admission/discharge• Weight• DRG – diagnosis-related group• Available since 2001 NHDS
– Source of Admission– Type of Admission
2002 NHDS
445 hospitals and 327,000 discharges were sampled
Weighted number of discharges was 33.7 million
Weights
• Must use weighted data to obtain national estimates
• Each record has a weight
• Must calculate the sum the weights of the records – one line of programming
Estimates have standard errors
A standard error is the sampling variability that occurs by chance
because only a sample rather than the entire universe is surveyed
For more design information
Plan and Operation of the National Hospital Discharge Survey: 1988
Redesign Vital and Health Statistics, 1(39). 2000
http://www.cdc.gov/nchs/data/series/sr_01/sr01_039.pdf
Designing Injury Research Project
• Determine what injury definition will be used – all injuries or selected injuries – what ICD-9-CM codes
• Decide how you will count injury patients• Select the data you will report• Evaluate whether and how external cause codes
will be used
Injury definitions• Definition developed by injury experts, e.g. the State and
Territorial Injury Prevention Directors Association (STIPDA) reported in Consensus recommendations for using hospital discharge data for injury surveillance, 2003
• Use existing categorization of codes – e.g. the Barell Matrix which defines injuries by type and body region in Injury Prevention, 8, 2002
• Injury and Poisoning Chapter of the ICD-9-CM – codes 800 to 999 – includes “true injuries” and “medical injuries”
• Have a specific ICD-9-CM code(s)
• Be relatively common in hospitalized patients or you will have to combine data over multiple years in order to get reliable estimates
Diagnoses selected for study should
Reliability • To be reliable, estimates must be based on at
least 30 records, and have a relative standard error of less than 30 percent - these usually produce weighted estimates of less than 5,000
• Estimates based on 30-59 records may be
unreliable and should be used with caution – these usually produce weighted estimates from 5,000-9,000
Counting injury patients
If you want the number of patients hospitalized because of an injury – count the first-listed diagnoses
If you want the number of hospitalized patients who have one or more injuries – count any-listed diagnoses
Counting injuries
If you want the number of injuries and not the number of patients – count all-listed diagnoses
Hospital discharges with fractures, 2002
Principal or first listed
Any listed All listed
995,000
1,387,000
1,609,000
Injury chartbook includes the following data
• Injury discharges by age and sex• Type and body region of injuries• Average number of diagnoses• Days of care/average length of stay• Expected source of payment• Discharge disposition• Percent with, and types of, external cause codes
Choices involving external cause codes
• Evaluate the percent of injury patients with external cause codes – how complete is it?
• If you decide you will use external cause codes, will you use just the first code or all codes?
• Consider using a previously developed categorization, e.g. - the External Cause of Injury Matrix
Challenges in studying trend data
• Coding changes over time
• Size and availability of data files
• Presenting the data
• Interpreting the data
Different versions of the International Classification of
Diseases
• 8th revision used 1970-78
• 9th revision used 1979-2004 – with addenda since 1986
Size and availability of NHDS data
• Single year files can be downloaded from the NHDS website and unzipped using free software. These include DRG’s.
• Multiple year files are on CD’s, rather than our website, due to their large size. They can be obtained by calling our office (301-458-4321). They do not include DRG’s.
Presenting the data
Age adjustment - Eliminates the differences in observed rates that result from age differences in population composition
over time
Presenting the data
Use of the log scale - allows the presentation of estimates with a very
wide range on the same graphand
facilitates comparison of the percent change of estimates over time
Presenting the data
Measures of change
AAPC – Average annual percent change from 1979-2001
APC – Average percent change for 1979-2001
Interpreting the results
Health service system changes which contributed to the decrease in hospitalization overall and for injuries
• Reform of Medicare hospital payment• Increased utilization review of hospital care• Growth in managed care• Expansion and coverage of ambulatory
surgery
Interpreting the results
Injury prevention activities which contributed to the decrease in hospitalization for injuries
• Encouragement of the use of safety belts and helmets
• Safer automobiles (air bags)• Safer roads• Improvements in home and workplace safety• Poison control centers
For more information:
• Check our website www.cdc.gov/nchs/about/major/hdasd/nhds.htm
• Phone: 301-458-4321
• Fax: 301-458-4032• Email: [email protected]