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• 1. What is CKD?

  Chronic kidney disease (CKD) is a condition in which the kidneys are damaged and cannot filter blood as well as possible. This damage can cause wastes to build up in the body and lead to other health problems, including cardiovascular disease (CVD), anemia, and bone disease. People with early CKD tend not to feel any symptoms. The only ways to detect CKD are through a blood test to estimate kidney function and a urine test to assess kidney damage. CKD is usually an irreversible and progressive disease, and can lead to kidney failure over time if it is not treated. Once detected, CKD can be treated through medication and lifestyle changes to slow down the disease progression, and prevent or delay the onset of kidney failure. However, the only treatment options for kidney failure are dialysis or a kidney transplant. More information can be found in the CKD fact sheet.

  
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• 2. How is CKD defined?

  The GFR is most often estimated (eGFR) using creatinine levels and estimating equations such as the Modification of Diet in Renal Disease (MDRD) equation (Levey et al., 1999), which includes adjustments for individual characteristics such as age, sex, and race. Following are the staging criteria from the National Kidney Foundation’s KDOQI Clinical Practice Guidelines for Chronic Kidney Disease: Evaluation, Classification, and Stratification (National Kidney Foundation’s K/DOQI Clinical Practice Guidelines for Chronic Kidney Disease: Evaluation, Classification, and Stratification)
  -Stage 1: normal kidney function (eGFR ≥90 ml/min per 1.73 m2) and persistent (≥3 months) proteinuria
  Stage 2: mild reduction in kidney function (eGFR 60-89 ml/min per 1.73 m2) and persistent (≥3 months) proteinuria
  Stage 3: moderate reduction in kidney function (eGFR 30-59 ml/min per 1.73 m2)
  Stage 4: severe reduction in kidney function (eGFR 15-29 ml/min per 1.73 m2)
  Stage 5: kidney failure (eGFR <15 ml/min per 1.73 m2); CKD stage 5 requiring dialysis or transplant for survival is also known as end-stage renal disease (ESRD)

  CKD covers a broad range of severity of disease, from normal kidney function with some evidence of kidney damage but likely no symptoms, to complete kidney failure requiring dialysis or kidney transplantation.

  
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• 3. What are the stages of CKD?

  The GFR is most often estimated (eGFR) using creatinine levels and estimating equations such as the Modification of Diet in Renal Disease (MDRD) equation (Levey et al., 1999), which includes adjustments for individual characteristics such as age, sex, and race. Following are the staging criteria from the National Kidney Foundation’s KDOQI Clinical Practice Guidelines for Chronic Kidney Disease: Evaluation, Classification, and Stratification (National Kidney Foundation’s K/DOQI Clinical Practice Guidelines for Chronic Kidney Disease: Evaluation, Classification, and Stratification)
  
  -Stage 1: normal kidney function (eGFR ≥90 ml/min per 1.73 m2) and persistent (≥3 months) proteinuria
  
  Stage 2: mild reduction in kidney function (eGFR 60-89 ml/min per 1.73 m2) and persistent (≥3 months) proteinuria
  
  Stage 3: moderate reduction in kidney function (eGFR 30-59 ml/min per 1.73 m2)
  
  Stage 4: severe reduction in kidney function (eGFR 15-29 ml/min per 1.73 m2)
  
  Stage 5: kidney failure (eGFR <15 ml/min per 1.73 m2); CKD stage 5 requiring dialysis or transplant for survival is also known as end-stage renal disease (ESRD)

  CKD covers a broad range of severity of disease, from normal kidney function with some evidence of kidney damage but likely no symptoms, to complete kidney failure requiring dialysis or kidney transplantation.

  
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• 4. What is the purpose of the CKD Surveillance project?

  Chronic kidney disease (CKD) is an important public health problem and has been recognized as a national health priority. It is defined by the presence of kidney damage or reduced kidney function for a period of at least 3 months. The level of disease severity has been used to classify CKD into various stages, from persistent kidney damage only (stage 1) to mild reduction in kidney function (stage 2) to moderate to severe reduction in kidney function (stage 3 and 4). Stage 5 refers to the advanced stage of CKD also termed “kidney failure,” which can progress to end-stage renal disease (ESRD), a term that implies kidney failure has reached the point of requiring dialysis therapy or kidney transplantation to maintain life.

  Patients with CKD suffer considerable morbidity as well as high rates of mortality. Kidney disease consistently ranks within the top 10 causes of death in the nation. While progression to ESRD is a well-known and serious complication of CKD, it is now well-recognized that premature death and morbidity (especially cardiovascular morbidity) are far more frequent outcomes compared to ESRD. Despite the tremendous impact of CKD on health, quality of life, and health care costs, the United States has thus far not developed a comprehensive, systematic surveillance program to monitor this important condition. Such a system would help not only in documenting the burden of CKD and its risk factors in the U.S. population over time, but also in tracking the progress of our efforts to prevent, detect, and manage CKD and its complications. It would also provide the means for evaluation, monitoring and implementation of quality improvement efforts by both federal and non-federal agencies. The CKD Surveillance Project was designed and implemented to address these issues.

  
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• 5. What organizations are responsible for the CKD Surveillance project?

  This project was funded by the Centers for Disease Control and Prevention. The CDC CKD Surveillance Team consists of members from groups led by University of California, San Francisco [Neil Powe (PI), Tanushree Banerjee, Yunnuo Zhu, Delphine Tuot, Chi-yuan Hsu, Charles McCulloch, Deidra Crews, Raymond Hsu, Vanessa Grubbs, Kirsten Bibbins-Domingo], University of Michigan [Rajiv Saran (PI), Brenda Gillespie, William Herman, Vahakn Shahinian,  Hal Morgenstern, Michael Heung, Yi Li, Friedrich Port, Bruce Robinson, William McClellan, Jennifer Bragg-Gresham, Diane Steffick, Anca Tilea, Sai Dharmarajan, Patrick Albertus, Jerry Yee, Rajesh Balkrishnan, Kara Zivin, April Wyncott], and Centers for Disease Control and Prevention [Desmond Williams, Nilka Ríos Burrows (Technical Advisor), Mark Eberhardt, Paul Eggers, Linda Geiss, Juanita Mondesire, Bernice Moore, Priti Patel, Meda Pavkov, Deborah Rolka, Sharon Saydah, Sundar Shrestha, Larry Waller].
  
  Additionally, in various phases of the project we have asked an Advisory Group to provide feedback on our project. The Advisory Group consists of individuals from various organizations, including American Association of Kidney Patients (Gary Green), American Association of Pediatric Nephrology (Susan Furth), National Kidney Disease Education Program (Andrew Narva), National Kidney Foundation (Joseph Vassalotti), VHA National Program Director for Kidney Disease and Dialysis (Susan Crowley), Medical Education Institute (Dori Schatell), ASN CKD Advisory Group (Deidra Crews), AHA Council for the Kidney in Cardiovascular Disease (Adam Whaley-Connell, Edgar Lerma), KDIGO (David Wheeler, Bertram Kasiske), and Renal Physicians Association (Dale Singer).

  
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• 6. How were the measures for the CKD Surveillance system chosen?

  A modified two-step Delphi process, typically used to gain consensus on a particular subject, was utilized. Topics and priority measures were first selected by enumerating key broad topics and possible measures for a CKD surveillance system derived by expert opinion within the Steering Committee and with input from an external Advisory Group as well as literature review. The second step involved obtaining input from the Steering Committee and the external Advisory Group on the relative importance of each of these measures for a surveillance system; and prioritizing the top-ranked measures to pursue in the pilot and subsequent phases of the project.

  Through team collaboration and discussion with project consultants and the Steering Committee, six broad topics that should be addressed by a CKD surveillance system were identified: burden of CKD (prevalence/incidence), awareness of CKD, risk factors for CKD, health consequences in CKD, process and quality of care in CKD, and health care system capacity for CKD. Within these six topics, a comprehensive list of 136 possible measures was compiled and is regularly updated.

  
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• 7. How were the data sources for each measure in the CKD Surveillance system chosen?

  Understanding CKD in the United States includes estimating the population burden, but also following the medical care received to prevent and treat disease – with these goals in mind, we determined what types of data to include. Through internet research and inter-team discussions, we developed a comprehensive list of potential data sources for the highest-rated topics and measures, including: national government data, registries, population-based and prospective cohort studies, state & local health departments, private industry (e.g., national laboratories), and health care system data (including health care plans, VA hospital systems, health care networks, Medicaid, etc.). This list of potential data sources was divided between the teams for the purposes of data source investigation.

  For each data source, we conducted internal investigations involving internet and literature searches. This information was summarized into an overall evaluation of the data source quality and a list of selected measures that could potentially be addressed with the data. Contacts were identified at each source and the summarized information was sent to each contact along with a request for an informational interview.

  For those data sources that agreed to an interview and provided contact information for knowledgeable interviewees, we conducted a comprehensive, standardized telephone interview. The purposes of the interview were to: (i) confirm the availability of data for any of our highest-rated measures; and (ii) evaluate data source’s attributes relevant to surveillance, including: acceptability, data quality, representativeness, defined denominator, timeliness, sensitivity, stability, simplicity, and flexibility.

  We have stored the wealth of information obtained during our informational interviews into a searchable database of CKD data. This database can be used to produce reports on availability and quality of CKD data, either overall or for a specific measure, from a variety of sources. Detailed comments regarding specific measures and data source attributes from the interviews are also stored in the database.

  Using this database, we produced reports for each data source-measure combination, each of which provided some general information about the data source and a rating for each attribute. Additionally, we produced a summary of each data source overall. Both documents were sent to our Advisory Group, and group members were asked to rank the data sources on their overall quality, and also to provide ranks for up to three data sources per measure, specifically for each of the 30 pilot measures. These rankings were used to prioritize the order in which we would seek to obtain and analyze data for the pilot surveillance system. Learn more about CKD Data Sources.

  
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• 8. Why did you use so many different data sources?

  Because no one source is able to capture all the important elements needed to track CKD (burden of disease, awareness, risk factors, health consequences, processes of care, and health care system capacity), we used multiple data sources.

  
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• 9. When there is more than one estimate for a measure, which one should I use?

  When there is more than one estimate, the following issues should be considered. Comparisons of estimates across data sources should consider differences in sampling, population denominator, and definition of CKD. For example, the NHANES data provide a population-based estimate of CKD prevalence because everyone sampled was tested for CKD. In contrast, health care system estimates represent the prevalence of detectable CKD because many individuals in the system are not tested.

  Additionally, these data sources are derived from populations that are very different with respect to demographics and comorbid conditions. Therefore, estimates should be compared only when standardized as much as possible with respect to these factors. Throughout this web site, many data are presented overall and by demographic and comorbidity subgroups.

  For data from the health care system, estimates are particularly dependent upon the definition of the denominator. For example, for prevalence of CKD, the investigators chose a denominator consisting of patients who utilized outpatient services at least once in a given year. Alternative denominators considered included the entire enrolled population or only those for whom an outpatient serum creatinine was performed. The former approach (prevalence among enrolled population) is affected by a large proportion of those eligible for their medical care not actually using the health care system and is therefore likely to result in significant underestimation of prevalence. The latter approach (prevalence among those tested) may produce excessively high prevalence estimates due to bias by clinical indication for the testing.

  It should be considered that the reasons for testing in health care system data may at least partially account for differences in prevalence estimates, regardless of the denominator chosen. Over time, changes in practice and awareness may change both the percentage tested and prevalence estimates.

  
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• 10. What equation did you use to estimate GFR?

  While several equations exist to estimate GFR from serum creatinine, at the present time, the MDRD Study 4-variable, or MDRD-4, equation is the most widely utilized by researchers and health care practitioners for calculating eGFR. However, we are aware that researchers are leading efforts to improve upon the current equations and/or develop new estimating equations. The CDC CKD Surveillance System is ultimately flexible for migration to a new equation that is proven to supersede the MDRD-4 equation in precision, ease of use, acceptance by the health and scientific communities, and prognostic value. We remain alert to the evolving science and will consider new equations as they become available.

  
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• 11. What about repeated serum creatinine or urine protein measures?

  The official definition of CKD relies upon documentation of persistent kidney damage or impairment of kidney function via repeated testing, a minimum of three months apart. However in working with data sets, both research- and health care-based, we have found that this rigorous level of testing is not consistently available. While we have incorporated data with repeated measures wherever feasible, most CKD prevalence information presented in the CKD Surveillance System web site is derived from a single urine protein or serum creatinine test, with the obvious caveat that some prevalence estimates may be increased from the true population prevalence.

  
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• 12. Why is there no information on genetic markers of CKD?

  Genetic markers for CKD are still being identified and the reliability and clinical utility of these markers is being investigated. Should particular markers emerge as important predictors of CKD, we remain open to incorporating data on these markers. The flexibility of our system will allow the inclusion of any new important indicator, as well as the removal of any indicators that become obsolete.

  
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• 13. Where can I find more information?

  Further information on kidney disease for both patients and professionals can be found at Related Links.

  
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• 14. How can I get involved?

  Investigators for the CKD Surveillance System are always considering new topics and data sources for inclusion. If you have suggestions or questions, please contact us at Contact CDC-INFO.

  
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