Measurement of anti-thyroglobulin (TgAb) predicts interference in thyroblobulin (Tg) assays and is a tumour marker for differentiated thyroid cancer (dtc). However, there is poor concordance between methods. This is attributed to the nature of TgAb, the method and cut-off used. We have assessed concordance between three TgAb assays and investigated the cause of discordant results. TgAb was measured in sera from 606 different patients being treated for dtc using the Abbott Architect, Roche E170 and Siemens Immulite TgAb assays. Reference range or lower reporting limit was used to classify samples as negative or positive for each assay. These were: Abbott <4 and <2, Immulite <40 and <20 and Roche <46 and <10 KU/L respectively. The prevalence of TgAb ranged 6%55% depending on the method and cut off used. Concordance between assays was 45% using the lower reporting limit and 75% using reference ranges to classify samples. Positive interference by Tg in the Roche TgAb assay was noted (Tg>1000 ug/L was associated with measurable TgAb). To investigate, pooled serum (Tg and TgAb negative) was spiked with Tg reference material BCR457. Measurement of TgAb in serial dilutions of the pool showed a proportional relationship between Tg and TgAb using the Roche assay. Use of the Siemens reference range rather than the lower reporting limit resulted in misclassification of samples as TgAb negative. All specimens with TgAb between the reference range and lower reporting limit of the Siemens assay were positive by the Roche and Abbott assays. Measurement of TgAb in serial dilution of the MRC65/93 TgAb standard showed the Immulite was less sensitive than the Roche and Abbott assays. We have found poor concordance between the TgAb assays assessed and have identified Tg interference and assay sensitivity as causes of discordance.
Declaration of interest: There is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.
Funding: No specific grant from any funding agency in the public, commercial or not-for-profit sector.