Thyroglobulin and thyroglobulin antibodies: assay-dependent management consequences in patients with differentiated thyroid carcinoma

OBJECTIVES

International guidelines recommend fixed cut-off values for thyroglobulin (Tg). These cut-offs do not take potential assay differences into account. This study aimed to evaluate if different assays for Tg and Tg antibodies (TgAb) affect management guidance for differentiated thyroid cancer (DTC) patients.

METHODS

In 793 samples derived from 413 patients with DTC, Tg and TgAb were simultaneously measured with two immunometric assays: Immulite 2000XPi and Kryptor compact plus. In addition, a qualitative measurement for TgAb interference (recovery test) was performed on the Kryptor compact plus platform. The extent to which different assays lead to different classifications of response to therapy was evaluated when applying the current cut-offs for Tg.

RESULTS

Mean Tg concentrations were 37.4% lower with Kryptor as compared with Immulite. Applying guideline based cut-off values for Tg, 33 (4.7%) samples had a Tg-on concentration ≥1.0 μg/L with Immulite and <1.0 μg/L with Kryptor. Of the samples tested as TgAb+ with at least one assay (n=125), 68 (54.4%) samples showed discrepancy in TgAb status. Differences between Immulite and Kryptor measurements resulted in a change in the response to therapy classification in 94 (12.0%) measurements derived from 67 (16.2%) individual patients.

CONCLUSIONS

A substantial portion of DTC patients were classified differently dependent on which Tg and TgAb assays are used, when applying the cut-off values as defined in clinical guidelines. Such differences can significantly affect clinical management. In the context of large between-method variation, the recommended Tg cut-offs in guidelines should be used with wisdom rather than as fixed cut-offs.

Thyroglobulin antibody (TgAb) methods - Strengths, pitfalls and clinical utility for monitoring TgAb-positive patients with differentiated thyroid cancer

Thyroglobulin autoantibodies (TgAb) are detected at diagnosis or during treatment in approximately 25% of patients with differentiated thyroid cancer (DTC). When present, TgAb interferes with thyroglobulin (Tg) measurement causing falsely low or undetectable Tg immunometric assay (IMA) values that can mask disease. Guidelines mandate that every Tg test have TgAb measured simultaneously and quantitatively by immunoassay and not a recovery test. The propensity and magnitude of TgAb-Tg interference relates to both Tg and TgAb concentrations and the class of Tg method used. Because the TgAb trend reflects changes in thyroid tissue mass, TgAb concentrations serve as a surrogate post-operative DTC tumor marker. A rising, or de novo appearance of TgAb may indicate recurrence, whereas a progressive decline suggests successful treatment. This review focuses on the technical limitations of current TgAb methods, characteristics of TgAb interference with different classes of Tg method, and the clinical value of monitoring TgAb trends as a surrogate DTC tumor marker.

Discordance in thyroglobulin measurements by radioimmunoassay and immunometric assay: a useful means of identifying thyroglobulin assay interference

Background

Serum thyroglobulin (Tg) is useful for monitoring patients with differentiated thyroid cancer (DTC) but is limited by interference from anti-Tg antibodies (TgAb). We determined Tg assay discordance between a radioimmunoassay (RIA) and one of two immunometric assays (IMA) in DTC patients over a 9-year period to gauge assay performance against evidence of recurrent/progressive DTC.

Methods

Patients with DTC monitored for >1 year attending local clinics between September 2000 and January 2010 were included. All samples were analysed for Tg using both RIA and IMA. TgAb were measured on all Tg requests made after May 2006. Bias plots comparing RIA against IMA were established to calculate a 2-SD outlier limit. Clinical records were viewed to compare discordant Tg results against clinical evidence of recurrent/progressive DTC.

Results

Discordant Tg results were observed in 53/433 patients (12.2%). Four were discordant owing to a higher IMA result, one of which demonstrated recurrence. The remaining 49 patients demonstrated a disproportionately higher RIA result, of which four had recurrent/persistent disease. Twelve patients with a higher RIA result but no evidence of recurrence underwent thyrogen stimulation testing, which was negative in all 12. In many cases, assay discordance appeared more sensitive at indicating interference than direct measurement of TgAb.

Conclusions

Interference was evident with both Tg assays, such that neither could be solely relied upon to provide the correct result in the presence of TgAb. The concomitant measurement of Tg by RIA and IMA methods should be considered as an alternative to monitoring TgAb status.

Comparison of the influence of thyroglobulin antibodies on serum thyroglobulin values from two different immunoassays in post surgical differentiated thyroid carcinoma patients

Measurement of serum thyroglobulin (Tg) is a highly specific test in the management of patients with differentiated thyroid cancer (DTC) after surgical treatment. The aim of our study was to evaluate and compare Tg levels in these patients found by radioimmunoassay (RIA) and immunoradiometric assay (IRMA) and to assess the influence of Tg antibodies (TgAbs) on the values obtained for Tg concentration. Both Tg and TgAb were determined postoperatively in the serum of 71 DTC patients using RIA Tg-PEG (INEP) and Tg IRMA (CIS) for Tg, together with TgAb (CIS) for circulating endogenous anti-TgAbs. The obtained concentrations were evaluated statistically. We found a significant difference of Tg concentrations between paired samples from the IRMA and RIA, although the intermethod comparison yielded satisfactory concordance of the two assays (Spearman correlation coefficient -0.792). Positive TgAb was found in 28.2% of the serum samples analyzed. Spearman rank correlation analysis revealed a significant negative relationship between serum TgAb and Tg level measured by IRMA (P=0.02), but not by RIA (P=0.417). On the other hand, our clinical data revealed that 1/18 and 3/18 patients with proven lymph node metastasis had Tg values below the detection limit by RIA and IRMA assay, respectively. Their sera were TgAb positive. We concluded that RIA was less prone to influence of TgAb than IRMA. As the presence of TgAbs may interfere in Tg measurement irrespective of the method selected for determination, this should be considered during the clinical management of these patients.

Measuring thyroglobulin and thyroglobulin autoantibody in patients with differentiated thyroid cancer

Measurement of serum thyroglobulin is primarily used as a tumor marker in the postoperative management of patients with differentiated thyroid cancer. Unfortunately, the technical quality of current thyroglobulin assay methods varies and influences the clinical utility of this test. Two different methodologic approaches are used to measure serum thyroglobulin: the original competitive radioimmunoassay methodology and noncompetitive immunometric assay methods. Although the newer immunometric assays offer the technical benefits of eliminating the use of isotopes, using smaller specimen volumes, and having higher sensitivity potential, shorter turnaround times and the convenience of automation, immunometric assays also have a higher propensity for interference from both thyroglobulin autoantibodies and heterophilic antibodies, if present in the specimen. It is critical that physicians understand the technical limitations inherent in thyroglobulin measurement in order to effectively use this test for the postoperative management of patients with differentiated thyroid cancers.

Thyroglobulin reference values in a pediatric infant population

Few reports have addressed normal serum thyroglobulin (Tg) values in newborns and infants. In the present study, serum Tg was measured in 228 normal children (110 females and 118 males) aged from 3 to 180 days of life, all presenting normal age-related serum thyrotropin (thyroid-stimulating hormone [TSH]) values and negative anti-Tg and antithyroperoxidase antibodies. Serum Tg was measured by Radioimmunoassay (RIA) (two methods) and Immunometricassay (IMA) (three methods). Mean Tg values measured by the five methods exhibited among-method biases, although a significant positive correlation was observed. Serum Tg levels measured by the five methods showed a correlation with age, but not with TSH or gender. During the first days of life, relatively high mean Tg levels were observed, which progressively decreased until they reached a plateau. Therefore, with the aim of establishing reference values, the population was separated into two age groups: from 3 to 15 days of life (group A) and from 16 to 180 days of life (group B). Mean Tg concentration in group A was statistically higher than in group B. Tg centile distributions were calculated with the aim of establishing the normal levels of serum Tg for each method. We conclude that for a correct interpretation of serum Tg levels, the age and the methods used should be considered.

Comparison of immunoradiometric assays for determination of thyroglobulin: a validation study

In this study we compared and validated commercial immunoradiometric assays (IRMA) to determine thyroglobulin (Tg) levels in serum. From a set of 440 samples, 68 were selected to calculate the validation parameters and the clinical performance of the assays. The commercial kits evaluated were the Tg-CTK (DiaSorin), IRMAZenco Tg (ZenTech), and SELco-Tg (Medipan). We found that 21% of the collected samples were in the critical range of concentration. Detection limits were calculated as being below 3 microg/L. Intra- and inter-reproducibility were lower than 3.1% and 9.2%, respectively. Dilution and recovery studies provided quantitative determinations. Correlation regression coefficients from the results of the methods were obtained. The determined concentrations were compared with the clinical evidence of disease. Variation in the 125-iodine-labeled antibody concentration and control charts showed the robustness of the methods. Analysis time and the simplicity of the methods were also evaluated. Reliable Tg determination is important for monitoring patients with differentiated thyroid cancer (DTC), controlling other thyroid diseases, and assessing the quality of imaging techniques. A strategy for verification and comparison based on analytical parameters and clinical performance is proposed.

Antithyroglobulin antibodies in patients with differentiated thyroid carcinoma: methods of detection, interference with serum thyroglobulin measurement and clinical significance

Antithyroglobulin antibodies (TgAb) were measured using a chemiluminescent immunoassay (ICMA) and an agglutination test. TgAb laboratory and clinical interference with Tg measurements were assessed. The course of TgAb concentration and disease status were compared during 3 years after initial treatment. The agglutination test failed to detect all titers < 10IU/mL (ICMA). Interference from TgAb was common at high titers, but even low antibody titers (< 5IU/mL) were able to interfere with Tg measurement. Cases of distant metastases with undetectable Tg (by IRMA) and those apparently free of disease and without thyroid remnants with Tg> 2ng/ml (by RIA) were identified among patients with TgAb. The exogenous Tg recovery test was normal (> 80%) by the two methods in 22% of patients with TgAb and confirmed laboratory interference. Absence of reduction in TgAb levels was a marker of persistent disease. In conclusion, TgAb should be determined by immunoassays; interference with Tg measurements occurred mainly but not always at high concentrations, with a normal Tg recovery test not excluding this interference. The behavior of TgAb is related to disease persistence or cure.

Thyroglobulin: a specific serum marker for the management of thyroid carcinoma

Thyroglobulin measurements in tissue and serum play an integral role in the evaluation of patients who have thyroid cancer. Immunohistochemical detection of thyroglobulin in surgical specimens is useful in the differential diagnosis of tumors of unknown origin; however, the most important application of thyroglobulin measurement in clinical practice is in the postsurgical management of differentiated thyroid cancer. Serum thyroglobulin is a highly specific and sensitive tumor marker for detecting persistent or recurrent thyroid cancer and for monitoring clinical status. The reappearance of circulating thyroglobulin after total thyroid ablation is pathognomonic for the presence of tumor. The measurement of thyroglobulin in serum is challenging, however, and several analytical problems limit assay performance. Thyroglobulin autoantibody interference is a particularly significant concern that requires all thyroglobulin samples to be screened for their presence. No immunoassay is totally free from interference by thyroglobulin autoantibodies. Measurement of thyroglobulin mRNA to detect circulating tumor cells may help to overcome some of the limitations of current protein-detection methods; serum thyroglobulin will continue to remain the "gold standard." The complex functional features of thyroid carcinomas make sole reliance upon any one diagnostic technique, including thyroglobulin assessments, potentially misleading. Thyroglobulin measurements are a critical component of a multifaceted diagnostic approach to this disease.

Discordant serum thyroglobulin results generated by two classes of assay in patients with thyroid carcinoma: correlation with clinical outcome after 3 years of follow-up

BACKGROUND

Serum thyroglobulin measurement is an integral part of monitoring patients with thyroid carcinoma, but analytic problems pose serious difficulties in the utility of this test.

METHODS

Between 1997 and 1998, serum samples from 83 patients with differentiated thyroid carcinoma were collected. Serum thyroglobulin was assayed by both radioimmunoassay and by an immunoradiometric assay. The disease status of patients with discordant serum thyroglobulin results was assessed in June 2001. Therefore, the predictive value of a single thyroglobulin measurement was assessed by evaluating the clinical status of patients 3 years later.

RESULTS

Discordant serum thyroglobulin results were noted in 17 (20.4%) patients. Of the 17 patients with discordant results, 16 had adequate clinical follow-up data. Of these 16 patients, 11 patients had detectable levels of serum thyroglobulin by immunoradiometric assay (range, 1.4-350 microg/L) whereas levels were undetectable by radioimmunoassay (< 1 microg/L). All 11 patients had evidence of metastases 3 years later. Two patients had undetectable serum thyroglobulin levels using the immunoradiometric assay (< 1 microg/L), whereas they had detectable levels using radioimmunoassay (serum thyroglobulin 7.2-30 microg/L). The serum samples from both patients had normal recoveries and positive antithyroglobulin antibodies. Both patients developed metastases 3 years later.

CONCLUSIONS

False-negative serum thyroglobulin results were significantly higher with the radioimmunoassay method compared with the immunoradiometric assay. The immunoradiometric assay is more reliable than the radioimmunoassay, particularly in patients who have no thyroglobulin antibodies. This finding is novel in that traditional immunoradiometric assay systems compared with radioimmunoassays usually have a higher incidence of false-negative results when assessed against clinical status. The immunoradiometric assay is subject to false-negative results in some patients with thyroglobulin antibodies, even when recovery experiments indicate the absence of interference. Thyroglobulin antibodies should be measured in all patients with differentiated thyroid carcinoma and if positive, results should be interpreted with extreme caution.