[Effects of venous compression for blood sampling on the results of in vitro tests of thyroid function (author's...

In a group of subjects with a normal as well as pathological thyroid function there was a significant increase in RT3U, serum-thyroxine-concentrations and TSH-levels but there were no significant changes in the T4-RT3U-indices and ETR-tests after 5 min of venous compression. The discordant results were obtained in 21 cases (26.9%) with the RT3U-test and in 4 cases (5,1%) with the T4-serum-level-determination, which may cause diagnostic errors. The non-standardized venous compression time causes a widening of the euthyroid zone which reduces the discrimination capabilities of these function tests. With TSH-determinations these deviations may be of diagnostic importance in the evaluation of TRH-TSH-test results. A standardized and minimum compression time when drawing blood samples is recommended in order to eliminate this potential source of diagnostic error.