title: Two novel equations to estimate kidney function in persons aged 70 years or older.
abstract: BACKGROUND
  In older adults, current equations to estimate glomerular filtration rate (GFR) are not validated and may misclassify elderly persons in terms of their stage of chronic kidney disease.
  
  
  OBJECTIVE
  To derive the Berlin Initiative Study (BIS) equation, a novel estimator of GFR in elderly participants.
  
  
  METHODS
  Cross-sectional. Data were split for analysis into 2 sets for equation development and internal validation.
  
  
  METHODS
  Random community-based population of a large insurance company.
  
  
  METHODS
  610 participants aged 70 years or older (mean age, 78.5 years).
  
  
  METHODS
  Iohexol plasma clearance measurement as gold standard.
  
  
  METHODS
  GFR, measured as the plasma clearance of the endogenous marker iohexol, to compare performance of existing equations of estimated GFR with measured GFR of the gold standard; estimation of measured GFR from standardized creatinine and cystatin C levels, sex, and age in the learning sample; and comparison of the BIS equations (BIS1: creatinine-based; BIS2: creatinine- and cystatin C-based) with other estimating equations and determination of bias, precision, and accuracy in the validation sample.
  
  
  RESULTS
  The new BIS2 equation yielded the smallest bias followed by the creatinine-based BIS1 and Cockcroft-Gault equations. All other equations considerably overestimated GFR. The BIS equations confirmed a high prevalence of persons older than 70 years with a GFR less than 60 mL/min per 1.73 m2 (BIS1, 50.4%; BIS2, 47.4%; measured GFR, 47.9%). The total misclassification rate for this criterion was smallest for the BIS2 equation (11.6%), followed by the cystatin C equation 2 (15.1%) proposed by the Chronic Kidney Disease Epidemiology Collaboration. Among the creatinine-based equations, BIS1 had the smallest misclassification rate (17.2%), followed by the Chronic Kidney Disease Epidemiology Collaboration equation (20.4%).
  
  
  CONCLUSIONS
  There was no validation by an external data set.
  
  
  CONCLUSIONS
  The BIS2 equation should be used to estimate GFR in persons aged 70 years or older with normal or mild to moderately reduced kidney function. If cystatin C is not available, the BIS1 equation is an acceptable alternative.
  
  
  BACKGROUND
  Kuratorium für Dialyse und Nierentransplatation (KfH) Foundation of Preventive Medicine.
authors.affiliation: Charité University Medicine, Campi Virchow, Mitte, Germany. elke.schaeffner@charite.de
authors.forename: Elke S
authors.initials: ES
authors.lastname: Schaeffner
authors.forename: Natalie
authors.initials: N
authors.lastname: Ebert
authors.forename: Pierre
authors.initials: P
authors.lastname: Delanaye
authors.forename: Ulrich
authors.initials: U
authors.lastname: Frei
authors.forename: Jens
authors.initials: J
authors.lastname: Gaedeke
authors.forename: Olga
authors.initials: O
authors.lastname: Jakob
authors.forename: Martin K
authors.initials: MK
authors.lastname: Kuhlmann
authors.forename: Mirjam
authors.initials: M
authors.lastname: Schuchardt
authors.forename: Markus
authors.initials: M
authors.lastname: Tölle
authors.forename: Reinhard
authors.initials: R
authors.lastname: Ziebig
authors.forename: Markus
authors.initials: M
authors.lastname: van der Giet
authors.forename: Peter
authors.initials: P
authors.lastname: Martus
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D015415:Biomarkers / Q000097:blood; D003404:Creatinine / Q000097:blood; D003430:Cross-Sectional Studies; D055316:Cystatin C / Q000097:blood; D005260:Female; D005919:Glomerular Filtration Rate; D006801:Humans; D007472:Iohexol / Q000378:metabolism; D008297:Male; D055641:Mathematical Concepts; D008657:Metabolic Clearance Rate; D051436:Renal Insufficiency, Chronic / Q000097:blood / Q000175:diagnosis / Q000503:physiopathology*
chemical_list: D015415:Biomarkers; D055316:Cystatin C; D007472:Iohexol; D003404:Creatinine