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.
Keywords: Dye-binding method; albumin assays; interference; nephrotic syndrome; α2-macroglobulin.
© The Author(s) 2015.