Comparison of Ultrasensitive and Highly Sensitive Assay to Predict Stimulated Thyroglobulin Levels Using Unstimulated Levels in Differentiated Thyroid Cancer Patients.

Background: Thyroglobulin (Tg) measurement is an essential aspect of monitoring for differentiated thyroid cancer (DTC) patients. This study compared the performances of ultrasensitive Tg (ultraTg) and highly sensitive Tg (hsTg) assays in predicting stimulated Tg levels without thyroid-stimulating hormone stimulation.

Methods: Overall, 268 DTC patients who had undergone total thyroidectomy and either radioiodine treatment or I-123 diagnostic scanning were included. Unstimulated and stimulated Tg levels were measured using hsTg (BRAHMS Dynotest Tg-plus) and ultraTg (RIAKEY Tg immunoradiometric assay) assays. Correlations of each assay with the ability of unstimulated Tg levels to predict stimulated Tg ≥1 ng/mL were analyzed.

Results: hsTg and ultraTg showed a strong correlation (R=0.79, P<0.01); the correlation was weaker in Tg antibody-positive patients (R=0.52). UltraTg demonstrated higher sensitivity in predicting stimulated Tg ≥1 ng/mL compared with hsTg. The optimal cut-off for ultraTg was 0.12 ng/mL (sensitivity, 72.0%; specificity, 67.2%). hsTg at 0.105 ng/mL had lower sensitivity (39.8%) but higher specificity (91.5%). Eight discordant cases with low hsTg (<0.2 ng/mL) but elevated ultraTg (>0.23 ng/mL) were identified; three developed structural recurrence within 3.4 to 5.8 years. Two patients had an excellent response according to hsTg but an indeterminate or biochemical incomplete response according to ultraTg.

Conclusion: UltraTg demonstrated higher sensitivity in predicting positive stimulated Tg levels and potential recurrence compared with hsTg. However, its lower specificity may lead to more frequent classifications of biochemical incomplete response. UltraTg may be beneficial in clinically suspicious cases where hsTg falls below the cut-off, but its broader applicability requires further investigation.