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[Thyroglobulin assay ambiguities]
1Laboratoire universitaire de biophysique, URA CNRS 1173, Faculté de Médecine, Strasbourg.
Insights
Thyroglobulin immunometric assays show significant variability due to lack of standardized reference material and interference from autoantibodies. Results require careful interpretation considering autoantibody presence for accurate clinical assessment.
Area of Science:
- Clinical Chemistry
- Immunoassay Technology
- Endocrinology
Background:
- Immunometric assays (IMA) for thyroglobulin have largely replaced radioimmunoassays but present persistent challenges.
- Lack of widespread use of human thyroglobulin reference material (CRM 457) contributes to significant inter-kit variability in results.
Purpose of the Study:
- To highlight the limitations and potential interferences in current thyroglobulin immunometric assays.
- To emphasize the need for improved standardization and interpretation of thyroglobulin measurements.
Main Methods:
- Discussion of common issues in IMA, including the high-dose hook effect and interference from thyroglobulin autoantibodies.
- Evaluation of the limitations of the recovery test for detecting autoantibody interference.
Main Results:
- The high-dose hook effect can falsely lower thyroglobulin levels in high-concentration samples.
- Autoantibody interference, though reduced by antibody selection, persists and is not reliably detected by standard recovery tests.
- Thyroglobulin levels are lower in Graves' disease patients with autoantibodies, even with normal recovery tests.
Conclusions:
- Thyroglobulin assay results must be interpreted considering the presence or absence of autoantibodies.
- Development of assays measuring total thyroglobulin (free and autoantibody-bound) is needed to improve performance evaluation.
Abstract:
Thyroglobulin immunometric "sandwich" assays (IMA) have taken over competitive radioimmunoassays, but this assay remains problematic. A human thyroglobulin reference material (CRM 457) has been prepared but is not widely used. That constitutes the main cause of very marked between kit variability of thyroglobulin results. High-dose hook effect, which can falsely decrease the result of a serum with high concentration of thyroglobulin, is not exceptional in one-step assays and should be systematically checked. Selection of monoclonal antibodies with no cross-reactivity with anti-thyroglobulin autoantibodies or of polyclonal antibodies with very high affinity, have reduced the frequency of interference due to autoantibodies, but did not abolish it. Recovery test is used to detect such interference, but with insufficient sensitivity. In fact, recovery determination can be influenced by the nature of thyroglobulin added to the serum (not identical to endogenous thyroglobulin), by the delay of incubation of exogenous thyroglobulin and serum autoantibodies and by the amount of added thyroglobulin. In addition, recovery is often wrongly expressed as the observed/theoretic ratio of final concentrations instead of added concentrations. In untreated Graves' disease patients and despite normal recovery test, thyroglobulin measured by IMA is lower when anti-thyroglobulin autoantibodies are present. Consequently, thyroglobulin result should be interpreted in function of presence or absence of autoantibodies. Development of total (free and autoantibody bound) thyroglobulin assay would be useful to evaluate assay and recovery test performances.