Comparison of Two Thyroglobulin Immunoradiometric Assays on the Basis of Comprehensive Imaging in Differentiated Thyroid Carcinoma

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Abstract
The aim was to compare two thyroglobulin-immunoradiometric assays (Tg-IRMA) in the follow-up of patients with differentiated thyroid carcinoma (DTC) in order to set up interassay correlation, correlation to clinical background, and to determine whether a lower functional sensitivity (kit A: 0.5 ng/mL, kit B: 0.3 ng/mL) would allow an earlier detection of recurrences. Three hundred eight samples from 181 patients with DTC were investigated. The clinical interpretation of the Tg-IRMA results was based on comprehensive imaging and the clinical history before and during the study period. Groups were formed against this background and against the thyrotropin (TSH) levels of the samples (LT4- on and LT4-off). During a follow-up period that lasted until September 1998, the clinical situation was reevaluated in order to determine any changes in the patients' clinical status. The two assays presented a good interassay correlation of 0.838. Both assays had a high and comparably good sensitivity in the detection of recurrence of malignancy or distant meastases. Patients in remission had, in most cases, nonmeasurable or Tg values below 1 ng/mL. Kit B presented slightly measurable Tg results in a larger number of patients in remission; however, during the follow-up most of these slightly measurable Tg results were not reproducible, thus being most likely artifacts. Consequently, the functional sensitivity of 0.3 ng/mL of kit B showed no advantages in terms of an earlier tumor detection and seems to be unacceptably low. Negative consequences may be an increase in the number of investigations during the follow-up, which may be disconcerting for both the clinicians and the patients.