DISCREPANCIES BETWEEN THYROTROPIN (TSH) MEASUREMENT BY 4 SENSITIVE IMMUNOMETRIC ASSAYS

A comparison of four different commercial immunometric thyrotropin (TS H) assays (Amerlite R TSH-30 Ultrasensitive assay from Kodak, BeriLux R hTSH from Behring Werke, Delfia R hTSH Ultra from Wallac and IMX R U ltrasensitive hTSH from Abbott) was made by measuring serum TSH in 81 consecutive patients referred to hospital for various reasons with a s erum TSH less than 0.8 mIU/l in the IMX assay. The analytical and func tional assay sensitivities of each of the assays were analysed. Even t hough three of the methods had a sensitivity corresponding to third ge neration assays, we could only demonstrate an overall agreement of ser um TSH when comparing two of the kits. The measurements in Delfia Ultr a and Berilux showed good agreement (P=0.7, paired t-test and bias=0.0 03 mIU/l), while the comparisons between the other assays showed diffe rent measurements (P < 0.00001, paired I-test and bias more than 0.07 mIU/l). Differences in the calibrators used in the assays might explai n some of the discrepancy, although all methods were calibrated accord ing to the same international standard. Also, differences in the speci ficity of the TSH monoclonal antibodies used in the assays might be an evident explanation and further studies of the specificity of the mon oclonal antibodies are needed. An international collaborative study to clarify reasons for the differences between the TSH assays and to sta ndardize the measurements is recommended. (C) 1997 Elsevier Science B. V.