Factors affecting T3 and T4 proficiency testing.

Problem areas within a proficiency testing (PT) program are performance evaluation and sample stability. The different units used in the various T3 uptake methodologies make performance evaluation complex. To facilitate this evaluation, a normalization method for T3 uptake performance evaluation has been developed. Sample stability studies for T3 uptake indicate that, at room temperature, sample values increase after storage for about seven days. Room temperature sample stability studies for T4 using a competitive protein binding (CPB) method indicate that the apparent T4 content of pooled serum increases after about one week. Fatty acids are shown to be an interfering substance in the T4 CPB method as well as the T4 radioimmunoassay (RIA) method. This interference increases with a decrease in carbon chain length from C18 to C12 and with an increase in unsaturation of fatty acids. The B/B0 ration for arachidonic acid at a concentration of 0.48 micronMoles per tube is 17.4 in a CPB method and 87.1 in a radioimmunoassay method indicating that the greater effect is in the CPB method. The increase in T3 uptake values are probably also due to the interfering effect of fatty acids.