Different Assay Evaluated for Thyroglobulin and Thyroglobulin Antibodies

Measurement of serum thyroglobulin (Tg) has an essential role in follow-up monitoring of patients with differentiated thyroid cancer (DTC). Tg measurements in combination with a neck ultrasound have a high sensitivity for detecting persistent or recurrent disease in patients previously treated with thyroidectomy and radioactive iodine (RAI) ablation.

Currently, most laboratories have adopted an automated immunometric assay for measurement of Tg. In recent years the diagnostic sensitivity has been improved with the introduction of assays with functional sensitivities (FS) ≤0.1 μg/L, also referred to as high-sensitive Tg assays (hs-Tg). The measurement of Tg is complicated by multiple factors.

Internal Medicine Specialists at the Erasmus Medical Center (Rotterdam, The Netherlands) and their colleagues aimed to evaluate if different assays for Tg and Tg antibodies (TgAb) affect management guidance for differentiated thyroid cancer (DTC) patients. In 793 samples derived from 413 patients with DTC, Tg and TgAb were simultaneously measured with two immunometric assays.

Serum Tg was measured with two immunometric assays: Immulite 2000XPi (Siemens Healthcare Diagnostics Inc, Tarrytown, NY, USA) and Kryptor compact plus (B.R.A.H.M.S Thermo Fischer Scientific, Hennigsdorf, Germany). In addition, a qualitative measurement for TgAb interference (recovery test) was performed on the Kryptor compact plus platform. The extent to which different assays lead to different classifications of response to therapy was evaluated when applying the current cut-offs for Tg.

The scientists reported that the mean Tg concentrations were 37.4% lower with Kryptor (mean 39.5 μg/L) as compared with Immulite (mean 63.1 μg/L). Applying guideline based cut-off values for Tg, 33 (4.7%) samples had a Tg-on concentration ≥1.0 μg/L with Immulite and <1.0 μg/L with Kryptor. Of the samples tested as TgAb+ with at least one assay (n=125), 68 (54.4%) samples showed discrepancy in TgAb status. Differences between Immulite and Kryptor measurements resulted in a change in the response to therapy classification in 94 (12.0%) measurements derived from 67 (16.2%) individual patients.

The authors concluded that a substantial portion of DTC patients were classified differently dependent on which Tg and TgAb assays were used, when applying the cut-off values as defined in clinical guidelines. Such differences can significantly affect clinical management. In the context of large between-method variation, the recommended Tg cut-offs in guidelines should be used with wisdom rather than as fixed cut-offs. The study was originally published online on January 28, 2022 in the journal Clinical Chemistry and Laboratory Medicine.

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