An interlaboratory and intralaboratory comparison of serum T4 methodologies

Interlaboratory comparison of serum thyroxine analyses

Approximately 300 clinical chemistry laboratories participated in a survey of measurements of the thyroxine (T4) concentration in 13 lyophilized serum specimens prepared by spiking a base serum pool with several different levels of T4. Of 35 commercially available kit methods, 4 kits were used by 30 or more laboratories, and 9 by 10 or more laboratories. Values obtained with the Abbott or Nuclear Medical laboratories, kits, which were used by one-third of all laboratories that named a specific kit, averaged 1 to 2 micrograms/dL greater than those obtained with the other methods. Within-run coefficients of variation were 10 to 20% for specimens containing less than 3 micrograms/dL of T4 and 3 to 6% for specimens with more elevated T4 levels.

Minimal effects of free fatty acids on the competitive protein-binding assay of serum thyroxine on reusable Sephadex columns

In rats; following oral administration of cream and intravenous heparin, plasma free fatty acids and triiodothyronine Sephadex uptake were elevated. Simultaneously apparent serum thyroxine, measured by the Tetrasorb-E kit, was increased, but not when measured using reusable Sephadex columns. Addition of oleate to a human serum albumin solution or to human serum produced similar effects; appreciable rises in apparent thyroxine were noted in the kit assay but only minor changes occurred when Sephadex columns were used. In the latter procedure almost all [14C]oleic acid added to serum was removed in the first barbital buffer wash, before application of the thyroxine-binding globulin solution. In the serum thyroxine assay on reusable Sephadex columns, there is minimal interference by free fatty acids because they are removed by the first barbital buffer wash.

Optimization of T assay: a model study

Factors involved in the optimization of competitive binding radioassays have been analyzed using a thyroxine (T4) radioassay as a model system. The effects of kinetic errors are minimized by the use of the lowest possible incubation temperature for the separation step and by the use of labeled T4 of high purity (which is not consistently available from commerical sources). The use of labeled T4 containing significant quantities of labeled triiodothyronine (T3) can lead to a marked increase in bias and a decrease in precision with relatively small errors in the separation step due to the relatively short half-life of the T3-binder complex. The determination of the dissociation rate constant (not to be confused with the affinity constant) allows one to make useful estimates of the tolerances that can be allowed in the separation step in order to achieve a desired level of accuracy and precison.

Semi-automated competitive protein binding analysis of serum thyroxine on reusable Sephadex columns and its advantages over radioimmunoassay

Competitive protein-binding analysis of serum thyroxine on small, reusable, Sephadex columns has been further studied and improved. The improved, semi-automated procedure results in reduced working time and costs. It has also been established that triiodothyronine crossreacts only 1/6 to 1/9 as well as thyroxine, and can be ignored because it represents only about 1/80 of the total serum iodothyronine content. The economic and methodological advantages of the improved method over radioammunoassay and other displacement assays are discussed.