Methods and Limitations
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Methodology

The U.S. Renal Data System (USRDS) is a surveillance system for end-stage renal disease (ESRD) funded by the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health. USRDS collects, analyzes, and distributes information from clinical and claims data reports to the Centers for Medicare and Medicaid Services (CMS) regarding patients being treated for ESRD. CMS reimburses most of the total cost of ESRD treatment in the United States, supplies most of the data used by USRDS, and provides expertise on data management. USRDS is estimated to include more than 93 percent of the ESRD population in the United States.

We defined ESRD-DM incidence as cases initiating treatment for ESRD (dialysis or kidney transplantation) and having diabetes listed as the primary cause of renal failure. Rates were calculated using two population estimates: 1) the resident U.S. population from the Census, and 2) the number of U.S. residents with diabetes obtained from the weighted sample of the civilian non-institutionalized population in the National Health Interview Survey (NHIS). Three-year moving averages were used to improve the precision of the estimated population with diabetes, and incidence was age-adjusted on the basis of the 2000 U.S. standard population.

For the series of maps, ESRD-DM incidence was also calculated using two population estimates: 1) state-specific estimates of the U.S. resident population during 1985 and 2002 from the Census, and 2) state-specific estimates of the U.S. population with diabetes during 1994 and 2002 from CDC's Behavioral Risk Factor Surveillance System (BRFSS). BRFSS is an ongoing, state-based telephone survey of the non-institutionalized population aged 18 years or older that provides state-specific information on behavioral risk factors and preventive health practices. Respondents were considered to have diabetes if they responded "yes" to the question, "Has a doctor ever told you that you have diabetes?" Women who indicated that they only had diabetes during pregnancy were not considered to have diabetes. Except for 1994 and 2002 (2-year averages), 3-year averages were used to improve the precision of the state-specific estimates of the population with diabetes, and at least two years of data were required to obtain the estimate. Incidence was age-adjusted on the basis of the 2000 U.S. standard population using two age groups (18-64 years and 65 years or older).

 

Limitations

The USRDS dataset is a living record of ESRD care in the United States, and is continually updated with new information on the ESRD population. Delays in the reporting of data account for the changes in the reported counts of incident patients from year to year as late information is added to the USRDS dataset.

Ascertainment of incident cases is incomplete because the data are for persons receiving ESRD treatment as reported to CMS and do not include patients who die of ESRD before receiving treatment and those who are not reported to CMS. Because the incidence of ESRD-DM was defined in terms of initiation of ESRD treatment, changes in incidence may have been due to changes in factors other than disease incidence. These include changes in treatment and care practices, greater recognition of the etiologic role of diabetes in ESRD, better access to ESRD treatment or acceptance of treatment, or a combination of these factors.

The count of new ESRD patients for 1993 was lower than expected from prior trends and persisted even with the usual updating done by USRDS and CMS. According to USRDS, a compensatory over count of new patients appeared to occur in 1994, and presumably, some of these patients were truly incident in 1993. In addition, a new Medical Evidence form introduced in 1995 and required for all new dialysis patients is a source of duplicate records as non-Medicare patients become Medicare-entitled. However, USRDS continues to work with both CMS and the ESRD networks to improve the accuracy of the data.

In 1996, the NHIS estimate of the number of U.S. residents with diabetes was unusually low. Relative to 1995, the 1996 NHIS sample size was reduced by approximately 25% in the first and second quarters and by approximately 50% in the third and fourth quarters. Beginning in 1997, the NHIS survey methodology was changed; instead of asking a one-sixth sub sample of respondents whether during the preceding 12 months a family member had diabetes, all sampled adult respondents were asked whether a health professional had ever told them they had diabetes. The low estimate in 1996 and the survey redesign in 1997 resulted in a large increase in the number of people with diagnosed diabetes between 1996 and 1997.

CMS began collecting Hispanic ethnicity data in 1995 and NHIS began collecting it in 1997. Therefore, for ESRD-DM incidence trends, race and ethnicity were considered independently. Persons who identified themselves as black or white might be Hispanic or non-Hispanic, and persons who identified themselves as Hispanic might be of any race. Diabetes data from Puerto Rico were not reported to BRFSS until 1996. Both NHIS and BRFSS underestimate the true prevalence of diabetes because about one-third of persons with diabetes do not know they have it. Since BRFSS is a telephone survey, bias may be introduced because households without telephones are not included.

 

References

  1. United States Renal Data System. Annual data report, 2004. Minneapolis, MN: USRDS Coordinating Center; 2004. Available at http://www.usrds.org/adr_2004.htm.*
  2. CDC. 1997 National Health Interview Survey (NHIS) public use data release: NHIS survey description. Hyattsville, MD: US Department of Health and Human Services, CDC; 2000. Available at ftp://ftp.cdc.gov/pub/health_statistics/nchs/dataset_documentation/nhis/1997/srvydesc.pdf.
  3. Nelson RG, Knowler WC, Pettitt DJ, Bennett PH. Kidney diseases in diabetes. In: Diabetes in America. 2nd ed. Harris MI, Cowie CC, Stern MP, Boyko EJ, Reiber GE, Bennett PH, eds. Washington, DC: US Department of Health and Human Services, National Institutes of Health; 1995. DHHS publication no. (NIH)95-1468; 361--6.

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