BACKGROUND: Thyroglobulin (Tg) assays are used to monitor patients after ablative therapy for differentiated thyroid carcinoma (DTC). Detectable Tg suggests persistence or recurrence of disease. However, even with very sensitive assays, false positive or false negative results are likely to occur depending on the presence of positive or negative assay bias. METHODS: We compared 6 different Tg immunometric assays in 53 treated DTC patients in whom serum Tg during TSH-suppression therapy was below an earlier established clinical decision limit of 3 pmol/l (2 ng/ml) by our routine Tg assay, which was one of these 6. Serum Tg was measured before and after administration of recombinant human TSH (r-TSH). Assay bias and confidence intervals for each assay were calculated from the pre-r-TSH Tg values in the patient subgroup in which no significant rise after r-TSH was observed by any method. RESULTS: Upper reference limits based on confidence intervals instead of functional sensitivity resulted in significant improvement of concordance between assays and prediction of Tg-rise after r-TSH. CONCLUSION: Confidence intervals in which assay bias are taken into account, form a better guideline for detection of possible persistence or recurrence of DTC than functional sensitivities.