title: Lower serum albumin level is associated with higher fractional excretion of creatinine.
abstract: BACKGROUND
  Creatinine clearance (Ccr) overestimates glomerular filtration rate (GFR) due to the tubular secretion of creatinine. It is known that fractional excretion of creatinine (FE-Cr) increases with decreasing GFR. Association of serum albumin level with the tubular secretion of creatinine was also reported previously. Alteration of FE-Cr may affect the performance of GFR estimating equations based on serum creatinine. Therefore, we analyzed the factors influencing FE-Cr and compared the performance of GFR equations in subjects stratified by serum albumin levels.
  
  
  METHODS
  Seven hundred and fifty-seven Japanese subjects were included. GFR was measured by inulin renal clearance. GFR and Ccr were measured simultaneously. FE-Cr was calculated as the ratio of Ccr to GFR. Multivariate analysis was performed to evaluate the factors influencing FE-Cr. Age, gender, GFR, body mass index (BMI), body weight, height and serum albumin level were analyzed as the parameters. Estimated GFR was calculated by Japanese GFR equations based on serum creatinine (Eq-cr), serum cystatin C (Eq-cys) and 5 variables including serum albumin (Eq-5var).
  
  
  RESULTS
  FE-Cr in subjects with serum albumin <3.0, 3.0-3.9 and ≥4.0 g/dl were 1.63 ± 0.48, 1.53 ± 0.55, and 1.40 ± 0.36, respectively. FE-Cr in subjects with serum albumin <3.0 or 3.0-3.9 g/dl were significantly higher than the value in subjects with serum albumin ≥4.0 g/dl. Multivariate analysis showed that GFR (p < 0.0001) and serum albumin level (p = 0.004) were independent parameters affecting FE-Cr. Biases of Eq-cr, Eq-cys and Eq-5var in subjects with serum albumin <3.0 g/dl were -9.5 ± 17.5, -0.7 ± 17.1 and -0.6 ± 14.8 ml/min/1.73 m(2), respectively. Eq-cr significantly overestimated GFR compared with Eq-cys or Eq-5var. Biases in subjects with serum albumin ≥4.0 g/dl were 6.4 ± 18.8, 2.0 ± 18.1 and 3.0 ± 18.3 ml/min/1.73 m(2), respectively. Eq-cr significantly underestimated GFR compared with Eq-cys or Eq-5var.
  
  
  CONCLUSIONS
  GFR and serum albumin level were independent parameters affecting FE-Cr. Alteration of FE-Cr according to the serum albumin levels may be one of the reasons of the bias of GFR equation based on serum creatinine.
authors.affiliation: Department of Functional Diagnostic Science, Osaka University Graduate School of Medicine, Suita, Osaka, 565-0871, Japan, horio@sahs.med.osaka-u.ac.jp.
authors.forename: Masaru
authors.initials: M
authors.lastname: Horio
authors.forename: Enyu
authors.initials: E
authors.lastname: Imai
authors.forename: Yoshinari
authors.initials: Y
authors.lastname: Yasuda
authors.forename: Tsuyoshi
authors.initials: T
authors.lastname: Watanabe
authors.forename: Seiichi
authors.initials: S
authors.lastname: Matsuo
mesh_terms: D000328:Adult; D000368:Aged; D015992:Body Mass Index; D003404:Creatinine / Q000097:blood*; D055316:Cystatin C / Q000097:blood; D005260:Female; D005919:Glomerular Filtration Rate / Q000502:physiology*; D006801:Humans; D007668:Kidney / Q000502:physiology*; D007677:Kidney Function Tests; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D012709:Serum Albumin / Q000378:metabolism*
chemical_list: D055316:Cystatin C; D012709:Serum Albumin; D003404:Creatinine