Clinical consequences of a change in anti-thyroglobulin antibody assays during the follow-up of patients with differentiated thyroid cancer.

OBJECTIVE: Thyroglobulin (Tg) quantitation by immunometric assays is compromised by anti-thyroglobulin antibodies (TgAbs), potentially resulting in falsely low Tg concentrations. TgAb screening is recommended when measuring Tg, but current TgAb immunoassays do not detect all possible TgAbs in circulation. We assessed the impact of a change in TgAb assay on apparent disease status of patients with differentiated thyroid carcinoma (DTC).

METHODS: Patients seen at the Mayo Clinic, Rochester, Minnesota, for follow-up of DTC, who had been tested using 2 different TgAb assays (Beckman Access and Roche Elecsys) were identified. Electronic medical records were reviewed to evaluate any impact the change in TgAb assay had on clinical disease status assessment and follow-up.

RESULTS: A total of 1,457 patients were tested using both assays. A change in TgAb status was found in 124 (8.5%) patients; a total of 117 patients who were TgAb-negative on the Beckman assay became TgAb-positive with the Roche assay. Additional testing was performed in 5 of these patients. Seven patients previously considered TgAb-positive were now TgAb-negative. In all 7 cases, physicians documented that they considered these patients now to be truly TgAb-negative and free of disease.

CONCLUSION: Discrepancies in TgAb status are seen when using different TgAb assays. Relying on Tg and TgAb measurements to determine disease status may lead to underestimation of residual cancer. A multimodal (clinical, biochemical, and radiologic) approach to follow up on patients with DTC should be continued, pending the development of Tg quantitation methods that are highly sensitive and not affected by TgAb interference.