ANALYTICAL PERFORMANCE OF FREE AND TOTAL THYROXINE ASSAYS

Excessive bias and imprecision are major analytical problems associate d with some assays for free and total thyroxine (T-4). Bias in free T- 4 methods is largely proportional to variations in serum T-4 binding. In direct methods, this is attributable to requirements for substantia l quantities of protein-bound T-4 to replace analytical losses of free T-4. In some total T-4 methods, bias is inversely proportional to the amount of serum T-4 binding and is attributable to the incomplete rel ease of serum protein-bound T-4. In others, bias is fixed and attribut able to inaccurate calibration. Manufacturers should report the bias i n their methods. Calibrations should be standardized, Imprecision vari es widely among methods, but is generally less for total T-4 methods t han for free T-4 methods. A consensus on quantitative analytical perfo rmance goals for free and total T-4 methods mould be helpful. Here, pe rformance goals are proposed, based in part on the best achievements o f current methods.