[Clinical evaluation of a new thyroglobulin immunoradiometric assay in the follow-up of differentiated thyroid carcinoma]

Formal and clinical comparison of a new 3 (rd) -generation-Tg-IRMA (3-G-IRMA; Dynotest Tg-plus) with a conventional Tg-IRMA (3-G-IRMA; SELco Tg-assay) for patients with differentiated thyroid carcinoma. In addition we evaluated, if thyroglobulin (Tg) levels above a specific threshold concentration indicate the need for further investigations for residual disease.Tg concentration of 105 sera of 93 consecutive patients with a differentiated thyroid cancer was determined with both assays and compared at different cut-off values (Dynotest Tg-plus: 0.2, 1, 2 ng/ml; SELco Tg-assay: 0.5, 1, 2 ng/ml) with the clinical results in respect to the corresponding TSH concentration.Tg concentration did not show any significant difference (SELco Tg-assay 0.5 ng/ml, Dynotest Tg-plus 0.2 ng/ml). The Tg-values of both assays correlated with 97%. However, correlation of recovery in both assays was small (40%). The sensitivities and specificities of both assays at different cut-offs and TSH values did not reveal significant differences. In patients with TSH concentration30 micro U/ml the functional assay sensitivity was superior to arbitrary cut-offs in the decision to start further evaluations.In our study neither formal nor clinical significant differences between two Tg-assays were found. In a hypothyroid patient (TSH30 micro U/ml, Tg concentration exceeding the functional assay sensitivity) further investigations for residual disease are warranted. Higher thresholds are of limited value, due to an unacceptably high rate of false negative results.