Studying Injuries Using the National Hospital Discharge Survey

1. Studying Injuries Using the National Hospital Discharge Survey Marni Hall, Ph.D. Hospital Care Statistics Branch, Division of Health Care Statistics

2. 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

3. Upcoming Chartbook Trends in Injury Hospitalization, United States, 1979-2001 by Melissa Heinen, Marni Hall, Manon Boudreault, and Lois Fingerhut

4. NHDS a national probability sample of short-stay non-federal hospitals - conducted every year since 1965provides data on discharges or hospitalizations – not individuals2002 data - now available2003 data - available in the winter

5. NHDS Design Three stage design Geographic Units Hospitals Discharges

6. Data Collection Automated – 40% Manual – 60%

7. Patient Data • Sex • Race • Age • Expected source of payment • Discharge status – including deaths

8. Facility Characteristics • Geographic region • Bed size • Ownership

9. Medical DataCoded according to the International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) • Diagnoses • Surgical and nonsurgical procedures

10. Additional Variables • Days of care • Month of admission/discharge • Weight • DRG – diagnosis-related group • Available since 2001 NHDS • Source of Admission • Type of Admission

11. 2002 NHDS445 hospitals and 327,000 discharges were sampled Weighted number of discharges was 33.7 million

12. 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

13. Estimates have standard errorsA standard error is the sampling variability that occurs by chance because only a sample rather than the entire universe is surveyed

14. 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

15. 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

16. 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”

17. Diagnoses selected for study should • 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

18. 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

19. Counting injury patientsIf you want the number of patientshospitalized because of an injury – count the first-listed diagnosesIf you want the number of hospitalized patients who have one or more injuries – count any-listed diagnoses

20. Counting injuriesIf you want the number of injuries and notthe number of patients – count all-listed diagnoses

21. Hospital discharges with fractures, 2002 1,609,000 1,387,000 995,000 Principal or first listed All listed Any listed

22. 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

23. 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

24. Challenges in studying trend data • Coding changes over time • Size and availability of data files • Presenting the data • Interpreting the data

25. Different versions of the International Classification of Diseases • 8th revision used 1970-78 • 9th revision used 1979-2004 – with addenda since 1986

26. 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.

27. Presenting the data Age adjustment - Eliminates the differences in observed rates that result from age differences in population composition over time

28. Presenting the data Use of the log scale - allows the presentation of estimates with a very wide range on the same graphandfacilitates comparison of the percent change of estimates over time

29. Presenting the data Measures of changeAAPC – Average annual percent change from 1979-2001APC – Average percent change for 1979-2001

30. 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

31. 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

32. For more information: • Check our website www.cdc.gov/nchs/about/major/hdasd/nhds.htm • Phone: 301-458-4321 • Fax: 301-458-4032 • Email: NHDS@cdc.gov