title: The bromocresol green assay, but not the modified bromocresol purple assay, overestimates the serum albumin concentration in nephrotic syndrome through reaction with α2-macroglobulin.
abstract: BACKGROUND
  The bromocresol green (BCG) assay is commonly used for measuring albumin (ALB), but is affected by α1- and α2-globulins, which are elevated in systemic inflammation. The modified bromocresol purple (mBCP) assay is another dye-binding method developed to overcome non-specific reactions. Concentrations of α2-macroglobulin, a major α2-globulin component, are increased in nephrotic syndrome (NS), but not in inflammation. There is little direct evidence that α2-macroglobulin affects BCG or mBCP assays.
  
  
  METHODS
  We measured serum albumin concentrations in 33 patients with NS and 13 reference healthy controls using BCG (ALBBCG) and mBCP (ALBmBCP) assays, and nephelometry (nALB) as a reference method. We also determined five specific proteins belonging to the α1- and α2-globulins by nephelometry. After adding purified α2-macroglobulin to albumin solutions, protein reactivity in these three assays was compared.
  
  
  RESULTS
  Nephrotic syndrome patients were categorized to tertiles according to nALB concentration. In all tertiles, ALBBCG was significantly higher than nALB, especially in the severe hypoalbuminemia group, in which α2-macroglobulin was 43-49% higher. By contrast, ALBmBCP and nALB were almost identical in all three groups. The difference between ALBBCG and nALB was positively correlated with the α2-macroglobulin concentration. In vitro, when α2-macroglobulin was added to solutions containing identical albumin concentrations, α2-macroglobulin dose-dependently increased ALBBCG, but not ALBmBCP.
  
  
  CONCLUSIONS
  In NS, α2-macroglobulin is a major factor for positive bias of ALBBCG, especially in patients with severe hypoalbuminemia. The mBCP assay is useful for measuring albumin concentrations in NS.
authors: Department of Clinical Laboratory Medicine, Juntendo University Graduate School of Medicine, Tokyo, Japan.
authors: Tsuyoshi
authors: T
authors: Ueno
authors: Department of Clinical Laboratory Medicine, Juntendo University Graduate School of Medicine, Tokyo, Japan sthiraya@juntendo.ac.jp.
authors: Satoshi
authors: S
authors: Hirayama
authors: Department of Clinical Laboratory Medicine, Juntendo University Graduate School of Medicine, Tokyo, Japan.
authors: Masami
authors: M
authors: Sugihara
authors: Department of Clinical Laboratory Medicine, Juntendo University Graduate School of Medicine, Tokyo, Japan.
authors: Takashi
authors: T
authors: Miida
mesh_terms: D001774:Blood Chemical Analysis / Q000379:methods*; D001961:Bromcresol Green / Q000737:chemistry*; D001962:Bromcresol Purple / Q000737:chemistry*; D005189:False Positive Reactions; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009404:Nephrotic Syndrome / Q000097:blood*; D012709:Serum Albumin / Q000032:analysis* / Q000737:chemistry; D000511:alpha-Macroglobulins / Q000032:analysis*
chemical_list: D012709:Serum Albumin; D000511:alpha-Macroglobulins; D001962:Bromcresol Purple; D001961:Bromcresol Green
keywords: Dye-binding method; albumin assays; interference; nephrotic syndrome; α2-macroglobulin