title: Matrix-dependent bias in total thyroxine measurement on the Beckman Access.
abstract: BACKGROUND
  Total thyroxine assays continue to be an integral part of thyroid status testing. We experienced a significant number of elevated total thyroxine values in patients with normal thyroid stimulating hormone and thyroxine binding globulin concentrations using the Beckman Access and hypothesized that these were due to a sample matrix effect.
  
  
  METHODS
  We compared the total thyroxine assays on the Beckman Synchron, Beckman Access, and Dade Dimension using individual patient specimens and two uniform matrices: a serum pool and an albumin-based matrix that were both supplemented with L-thyroxine to span the linear assay range.
  
  
  RESULTS
  Access total thyroxine values in individual patient specimens exhibited sporadic positive bias as high as 77 nmol/l (6 microg/dl) when compared to the Synchron and Dimension. The use of uniform matrices had little effect on the Synchron in comparison to the Dimension but significantly improved the agreement between the Access and the Dimension or Synchron as indicated by a statistically significant improvement in correlation coefficients.
  
  
  CONCLUSIONS
  The Access total thyroxine assay is prone to a variable and clinically significant positive bias that is mediated by a component of the sample matrix.
authors.affiliation: Pathology Service (113), Veterans Affairs Medical Center, 1030 Jefferson Ave., Memphis, TN 38104, USA. dennis.dietzen@med.va.gov
authors.forename: Dennis J
authors.initials: DJ
authors.lastname: Dietzen
authors.forename: Sandra F
authors.initials: SF
authors.lastname: Queen
authors.forename: Solomon S
authors.initials: SS
authors.lastname: Solomon
mesh_terms: D000328:Adult; D015982:Bias; D004868:Equipment Failure; D005189:False Positive Reactions; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D013959:Thyroid Diseases / Q000175:diagnosis; D013960:Thyroid Function Tests / Q000295:instrumentation* / Q000379:methods / Q000592:standards*; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood*; D013975:Thyroxine-Binding Proteins / Q000032:analysis
chemical_list: D013975:Thyroxine-Binding Proteins; D013972:Thyrotropin; D013974:Thyroxine