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Abstract Background Serum thyroglobulin (Tg) is a key biomarker used to detect disease recurrence in patients with differentiated thyroid cancers who have undergone complete thyroidectomy. Thyroglobulin antibodies (TgAb) can interfere with accurate measurement of serum Tg levels, producing falsely low results in automated immunoassays. Thus before measurement of Tg, TgAb levels are measured to determine if in-house immunoassays can be used or if the patient’s sample will need to be sent out for radioimmunoassay (RIA) or mass spectrometry methodologies, which are less sensitive to TgAb interference. We recently transitioned to new instrumentation and noted a sharp increase in Tg samples with TgAb interference that needed to be sent out when we used the limit of detection (LOD) of the Roche TgAb assay. This increases turnaround times for patients and increases laboratory costs. We hypothesized that the LOD of the Roche TgAb assay is below the level of interference in Tg immunoassays. To test this we compared the performance of TgAb immunoassays (IA) and Tg measurements by IA and RIA with the intention of proposing a more appropriate threshold for TgAb interference for the Roche TgAb assay. Methods We collected 121 serum specimens, which were above the LOD of the Roche TgAb (R-TgAb) assay, which is 10 IU/mL. These specimens were sent as part of standard care to USC for measurement of TgAb by Kronus (K-TgAb) and Tg by RIA. On these split samples we also measured Tg by Beckman Access (B-Tg). We compared the detection of TgAb by USC to varying thresholds of Roche TgAb measurements (10 IU/mL LOD, 20 IU/mL, and 115 IU/mL reference interval for autoimmunity), and assessed the agreement between Tg measurements by Beckman IA and USC RIA for different Roche TgAb thresholds. Analyses were performed in Prism Graphpad using linear regression. Results Qualitatively, the concordance for R-TgAb to RIA (reference) at 10 IU/mL was 31%, at 20 IU/mL was 90%, and at 115 IU/mL was 80%. Quantitative agreement between the B-Tg immunoassay and RIA for the TgAb thresholds of: TgAb 10 IU/mL gave an R2=0.97 for Tg comparison, 20 IU/mL gave an R2=0.99, and 115 IU/mL R2=0.99. Conclusions Prior studies have shown that TgAb immunoassays can produce significant qualitative and quantitative differences. In our study, we attempt to validate the use of the Roche immunoassay with a cutoff of 20 IU/mL to minimize interference between Tg and TgAb in order to accurately detect recurrence of thyroid cancer. We found that a TgAb cutoff of 20 IU/mL we maintained acceptable agreement between Tg methods indicating no significant interference up to 20 IU/mL. With the TgAb cutoff of 20 IU/mL, we would be able to reduce sendouts for this cohort from 121 to 42, reducing sendouts by 65% and allowing a faster turnaround time for a significant proportion of our patients.
Patel et al. (Tue,) studied this question.