Clinical Chemistry
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Clinical Chemistry 32: 108-115, 1986;
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Clinical Chemistry, Vol 32, 108-115, Copyright © 1986 by American Association for Clinical Chemistry

On the albumin dependence of measurements of free thyroxin. I. Technical performance of seven methods

G Csako, MH Zweig, C Benson and M Ruddel

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.


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N. D. Christofides, E. Wilkinson, M. Stoddart, D. C. Ray, and G. J. Beckett
Assessment of Serum Thyroxine Binding Capacity-dependent Biases in Free Thyroxine Assays
Clin. Chem., April 1, 1999; 45(4): 520 - 525.
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