Tan Tock Seng Hospital1
Thyroglobulin (TG) is a useful tumour marker in differentiated thyroid carcinoma. However its measurement is subject to interference by anti-thyroglobulin autoantibodies (ATG). The aims of this study were to investigate the impact of using either the lower limit of detection or the upper reference limit to define ATG positivity when performing TG measurements.
We perform paired TG and ATG measurement in all TG requests on a Beckman Coulter DxI-800 clinical chemistry analyser. The lower limit of detection for ATG is 0.9 IU/mL and the upper reference limit (URL) is 4.0 IU/mL. Results of all TG/ATG testing for the past 24 months were examined using Microsoft Excel. Cases were categorised based on ATG as undetectable (U: <0.9 IU/mL), detectable/ not raised (D: 0.9 – 4 mIU/mL) and raised (R: >4 IU/mL).
There were 854 paired TG and ATG measurements performed: 669 U, 92 D and 93 R. Median TG values for all 3 groups was 0.1 IU/mL but the 95th percentiles varied: U: 67.6; D: 22.2; R: 19.72 IU/mL. The % of TG results greater than 5 x URL was: U: 11.6%, D: 6.5%, R: 5.3%.
The similarity in result distributions between the ATG detectable/not raised and the ATG raised groups suggests that that both groups show evidence of negative interference on TG measurement and that laboratories should choose the lower limit of detection as the cutoff for defining ATG positivity when reporting TG measurements.