On the albumin dependence of measurements of free thyroxin. I. Technical performance of seven methods.
We evaluated three one-step (analog) and two two-step radioimmunoassay for free thyroxin (FT4), and a FT4 index calculated from the total T4 (TT4) and thyroxin-binding globulin (TBG) ratio for technical performance, for correlation with the reference FT4 method (equilibrium dialysis), and for dependence on TBG and albumin concentrations. The one-step methods (Amerlex, Coat-A-Count, and GammaCoat) showed greater precision than the two-step procedures (GammaCoat and Spiria). Results by the latter two techniques, however, correlated better with those by equilibrium dialysis than did those by the analog methods or by TT4/TBG. Only the GammaCoat two-step method had a slight but statistically significant (inverse) correlation with TBG concentration. All three analog methods and TT4/TBG showed a marked dependence on albumin concentration, whereas the Spiria technique showed only a slight dependence. Only equilibrium dialysis was independent of both the TBG and albumin concentration. Thus, despite their good precision, the analog (one-step) FT4 methods and the TT4/TBG approach cannot be expected to produce valid results when the concentration of albumin in serum is abnormally low or high.
Csako
G
G
Zweig
M H
MH
Benson
C
C
Ruddel
M
M
D003956:Dialysis; D005260:Female; D006801:Humans; D008297:Male; D011863:Radioimmunoassay; D011933:Reagent Kits, Diagnostic; D012709:Serum Albumin / Q000032:analysis*; D013974:Thyroxine / Q000097:blood*; D013975:Thyroxine-Binding Proteins / Q000032:analysis*
D011933:Reagent Kits, Diagnostic; D012709:Serum Albumin; D013975:Thyroxine-Binding Proteins; D013974:Thyroxine