title: Within-person variability in kidney measures.
abstract: BACKGROUND
  Our objective was to quantify short-term total within-person variability in standard and nontraditional kidney measures using national data.
  
  
  METHODS
  Repeated examination study of serum and urine kidney measures.
  
  
  METHODS
  Participants 18 years or older in the Third National Health and Nutrition Examination Survey (NHANES III) who had repeated blood and urine samples collected during visits occurring approximately 18 days apart.
  
  
  METHODS
  Standardized serum creatinine, standardized cystatin C, β-trace protein (BTP), β(2)-microglobulin (B2M), and urine albumin and creatinine. We calculated the within-person coefficient of variation (CV(w)), which includes both biological and analytical variability. We also evaluated the impact of variability on estimates of the prevalence of reduced estimated glomerular filtration rate and albuminuria.
  
  
  RESULTS
  Serum cystatin C level demonstrated the lowest short-term within-person variability (CV(w) = 6.8%). Serum creatinine and B2M levels (CV(w) = 7.6% and 8.4%, respectively) also had low variability. BTP level had the most variability of the serum markers (CV(w) = 11.6%). As expected, urine albumin and urine creatinine measurements showed high variability (CV(w) >30% for both); however, albumin-creatinine ratio performed much better than either measure alone, with CV(w) of 11.3%. The effect of short-term variability on the prevalence of reduced estimated glomerular filtration rate was moderate, with an ~20% lower prevalence when defined based on single measurements compared to repeated application of the same test approximately 18 days apart. Repeated testing for albuminuria had a larger effect, showing a 33% lower prevalence of albuminuria when repeated testing was applied.
  
  
  CONCLUSIONS
  Only 2 measurements available. General population with low prevalence of kidney disease.
  
  
  CONCLUSIONS
  Our results suggest that creatinine, cystatin C, and B2M levels have similarly low short-term variability. BTP level was more variable compared with the other serum filtration markers. Urine albumin and creatinine levels were highly variable and may benefit from repeated assessments to reduce the misclassification of albuminuria.
authors: Department of Epidemiology and the Welch Center for Prevention, Epidemiology and Clinical Research, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD 21287, USA. lselvin@jhsph.edu
authors: Elizabeth
authors: E
authors: Selvin
authors: Stephen P
authors: SP
authors: Juraschek
authors: John
authors: J
authors: Eckfeldt
authors: Andrew S
authors: AS
authors: Levey
authors: Lesley A
authors: LA
authors: Inker
authors: Josef
authors: J
authors: Coresh
mesh_terms: D000419:Albuminuria / Q000453:epidemiology / Q000652:urine*; D003404:Creatinine / Q000097:blood* / Q000652:urine*; D003430:Cross-Sectional Studies; D055316:Cystatin C / Q000097:blood*; D005260:Female; D005919:Glomerular Filtration Rate; D006801:Humans; D019746:Intramolecular Oxidoreductases / Q000097:blood*; D007668:Kidney / Q000502:physiology*; D007677:Kidney Function Tests; D054834:Lipocalins / Q000097:blood*; D008297:Male; D008875:Middle Aged; D011159:Population Surveillance; D015995:Prevalence; D012189:Retrospective Studies; D014481:United States / Q000453:epidemiology; D001613:beta 2-Microglobulin / Q000097:blood*
chemical_list: D055316:Cystatin C; D054834:Lipocalins; D001613:beta 2-Microglobulin; D003404:Creatinine; D019746:Intramolecular Oxidoreductases; C022466:prostaglandin R2 D-isomerase