Sérums de contrôle de qualité pour les immunodosages de thyroxine libre (T4 L) : congelés plutôt que lyophilisés ?

Important biases are reported between free thyroxine (FT4) determined by equilibrium dialysis (ED) and by most immunoassays when T4-binding capacity is decreased. Ten lyophilized and 32 frozen quality control (QC) sera were analyzed in the italian and french (lyophilized sera) and british (frozen sera) external quality assessment programs for FT4 using five immunoassays (Elecsys, ADVIA Centaur, Vitros ECi, Immulite and AxSYM). We determined the bias between the FT4 results reported with each immunoassay and FT4 measured in our laboratory with ED. We measured in each serum the concentration of T4-binding proteins and the T4-binding capacity estimated by the Elecsys T4-Uptake test (TU). Mean biases were less marked with frozen samples (from -9% to -23%) than with lyophilized samples (from -4% to -44%). Biases were correlated with TU decreasing (p < 0.0001). The biases appeared to be related to the processing of the QC sera causing a reduction in their T4 affinities. Frozen QC sera involving less processing seem to be more suitable for FT4 external quality assessment.