2)

2). Open in a separate window Fig. and DxI 800=0.29RIA-MAT 280+13.93 (serum TgAb) (Fig. 2). Open in a separate window Fig. 2 Correlation of serum thyroglobulin (A) and anti-thyroglobulin antibody (B) concentrations measured by the two assays (results within analytical measurement ranges). When serum Tg levels were categorized into 1 ng/mL, 1C10 ng/mL, and 10 ng/mL, the weighted Kappa statistic value for Tg was 0.884 (Table 2, top). Serum TgAb levels were compared based on the cut-off value for each assay, resulting in an 84.8% LPP antibody (56/66) negative agreement and 36.4% (12/33) positive agreement, with a total agreement of 68.7% (Cohen’s Kappa statistic, 0.231; Table 2, bottom). Table 2 Agreement of serum thyroglobulin and anti-thyroglobulin antibody between the two assays =0.992, =0.999, respectively) [11]. However, a major concern with the ICMA is the TgAb interference, which often results in falsely low or undetectable Tg Imipramine Hydrochloride concentrations [5,9] in and clinical studies [8,9,19]. Endogenous TgAbs bind to free Tg and conceal the Tg epitopes needed for recognition by signal monoclonal antibody reagents [18]. Nevertheless, we observed a good correlation between the two Tg assays using TgAb-positive and -negative samples, although both Imipramine Hydrochloride assays may not fundamentally overcome the TgAb interference. For the ICMA, a 2- or 3-fold difference has been reported between some assays [5,11]. This might result from differences in specificity for the capture and/or signal monoclonal antibody reagents or standardization, although the ICMA is standardized against the CRM-457 international standard material [5,11]. Because of high variability and low interchangeability among assays [5], the current guidelines for differentiated thyroid cancer developed by the ATA recommend serial measurement of serum Tg using the same assay in individual patients [20]. Changes in Tg assays, even if it is an ICMA, may result in abrupt serum Tg fluctuation and need to be cautiously considered for adequate decision making. Eight cases presented discrepant results in term of Tg levels (Table 2). Among them, only subtle variations in Tg concentrations were observed in seven cases (i.e. 10.8 ng/mL using the IRMA vs 9.56 ng/mL using the ICMA). These differences were clinically acceptable changes and may not alter the clinical implication. However, one case showed a considerable elevation of serum Tg concentration on the DxI 800 when compared with the results obtained by using the GAMMA-10 (29.37 ng/mL vs 1.2 ng/mL). Based on clinical information, there was no evidence Imipramine Hydrochloride of remnant or recurrent disease on two instances of thyroid ultrasonography with a decreasing trend of subsequent stimulated Tg. Thus, the value measured by the DxI 800 might have been falsely elevated. Serum TgAb was positive, even with a slight elevation in this patient. However, the presence of TgAb always interferes with serum Imipramine Hydrochloride Tg levels, resulting in an underestimation in the ICMA [5]. Therefore, overestimation of serum Tg by the DxI 800 might derive from another type of interference. Human anti-mouse antibody (HAMA) also affects serum Tg results, mainly in terms of false elevation, even though this rarely leads to underestimation of Tg concentrations [21]. This antibody can bind to animal antigens and form a complex between antibodies, which produces a false elevation of serum Tg results [21]. As the prevalence of HAMA ranges 1.5C3% in automated Tg assays, HAMA interference should be suspected in cases showing Tg results discordant with the clinical situation [21]. This study has several limitations. Neither the ICMA nor IRMA tested in this study was a gold standard, although they are commonly recommended for Tg measurement. As described above, all IMA methodologies are affected by the interference from TgAbs as well as HAMA. This interference is not as evident as in other assays, including RIA and the LC-MS method. However, they also do not fully overcome it yet. In the present study, we identified one case showing a falsely elevated serum Tg result with the DxI 800, which should be interpreted in the clinical context. The proportion of discordant results (1/99) seems to be acceptable because the prevalence of HAMA has been reported to be up to 3% in IMAs. Even though the IRMA may not serve as the gold standard test, as the most prevalent assay for serum Tg.