Multicentre comparison of free thyroid hormones immunoassays: the Immunocheck study.

Background: In order to evaluate whether there is still present a very large variability in measured values and analytical performance among different free thyroid (FT) hormone immunoassays, we considered the results derived from an External Quality Assessment (EQA) scheme, named Immunocheck.

Methods: The EQA Immonocheck study enrolled about 1000 participant laboratories, which measured the 54 quality control samples distributed in the three annual cycles (2007, 2008 and 2009). Participant laboratories produced a total of 30,476 results for FT3 and 31,351 for FT4, respectively.

Results: The results recovered during the EQA cycles allowed the estimation of assay imprecision (i.e., within-method and between-laboratories variability). On average, control samples with lower free triiodothyronine (FT3) and free thyroxine (FT4) concentrations showed higher imprecision values (CV%) than those with levels within or above the normal range. The agreement among the results produced by different methods was estimated by computing the percent differences (i.e., percent bias values) between the concentrations measured by the methods and the mean of all collected results (i.e., consensus mean).

Conclusions: The results of our study demonstrate that a large variability in measured values is still present among different free thyroid hormone immunoassays. Indeed, some immunoassays for both FT3 and FT4 measurement showed percent bias values compared to the consensus mean >20%. Laboratories should inform the clinicians about analytical performance and reference limits of the method used. Furthermore, the clinicians should avoid the use of different methods in the follow-up of patients.