The impact of quality control materials on the performance of an internal quality control system: 3. Experiences from S-triiodothyronine analysis.

In this subproject a method for S-triiodothyronine was used for evaluation of the specified system of internal quality control. Decisions on acceptance and rejection of series were based on results from a human pool with triiodothyronine concentration at the upper reference limit. In the parallel system specimens from patients and from healthy individuals were used in combination with artificial materials so that changes in accuracy, specificity, and detectability could be disclosed. The results were examined for false rejections and false acceptances. For all the seven rejections in 117 series due to a mean rule of 8 controls with a pfr of 0.01, the parallel system supported the decisions. In eight occasions the series were rejected due to a range rule with a pfr of 0.05; one rejection was confirmed by the parallel system and the remaining seven times out of the 117 series were close to the expected five percent of false rejections. A small systematic change in standardization of about eight percent was too small to be recognized by the quality control system. The specificity was not changed during the investigation period; all of the systematic changes were caused by variations in the standard materials. In conclusion the investigation confirmed the validity of the specified system of internal quality control.