Analytical performance evaluation of thyroid-stimulating hormone receptor antibody (TRAb) immunoassays

US-based, Prospective, Blinded Study of Thyrotropin Receptor Antibody in Autoimmune Thyroid Disease

CONTEXT

Bioassays provide information on the functionality of thyrotropin receptor antibodies (TSH-R-Ab) and thus may offer more clinical utility than binding assays.

OBJECTIVE

In this prospective, blinded, US-based study, the clinical performance of several TSH-R-Ab assays was compared.

SETTING

US endocrinology clinic.

SUBJECTS

One hundred sixty-two unselected, consecutive, well-documented patients with various thyroid diseases and healthy controls.

INTERVENTION(S)

Blinded TSH-R-Ab measurements.

MAIN OUTCOME MEASURE(S)

Sensitivity and specificity of 4 TSH-R-Ab assays.

RESULTS

The 4 TSH-R-Ab assays were negative in all 42 patients without autoimmune thyroid disease (AITD). In 104 patients with Graves' disease (GD), irrespective of the disease duration, TSH-R-Ab positivity was present in 65 (63%), 67 (65%), and 87 (84%) for the Cobas and Immulite binding assays and stimulatory TSH-R-Ab [thyroid-stimulating immunoglobin (TSI)] bioassay, respectively (TSI vs Immulite P < .0025, TSI vs Cobas P < .0009). Fifteen newly diagnosed GD patients were all positive in the TSI bioassay, but only 11 (73%) were positive in the Cobas and Immulite binding assays. Nine GD patients with biochemical subclinical hyperthyroidism were TSI-positive but Immulite- and Cobas-negative. Two GD patients were blocking TSH-R-Ab [thyroid-blocking immunoglobin (TBI)]-positive and TSI-negative, and the Immulite and Cobas were positive in both. Additional serum samples from AITD patients that consisted of 30 TBI-positive and 10 TSI-positive samples were blindly tested in the binding assays. Only 6 of the 10 TSI-positive samples were positive in both binding assays, and 30 and 28 of the TBI-positive samples were positive in the Cobas and Immulite assays, respectively.

CONCLUSION

Binding TSH-R-Ab assays are less sensitive than TSI bioassays and are not specific for stimulating antibodies. Measuring the function of TSH-R-Ab in a bioassay can provide useful information to clinicians.

Stability of Anti-thyroid Stimulating Hormone Receptor Antibody in Whole Blood and Serum: Caution Required for Reflective and Batch Testing

INTRODUCTION

Anti-thyroid stimulating hormone receptor antibody (TRAb) stability is stated as 7h at 20-25°C in the Roche Elecsys assay kit insert. The purpose of this study was to determine TRAb stability in whole blood and serum to assess the suitability of samples for reflective and weekly batch testing (with a single freeze-thaw cycle).

METHODS

In the first study, blood from n = 5 volunteers was used to assess: (1) stability in whole blood at room temperature up to 24h, and (2) stability in serum at 4-8°C up to 72h. In the second study, n = 21 patient samples were analysed in serum stored at 4-8°C for two and five days post-preliminary analysis.

RESULTS

There was a statistically significant decrease in TRAb concentration caused by the single freeze-thaw cycle of -8.9% ± 5.2% (p = 0.03). TRAb concentration decreased in whole blood between 0 and 24h by -16.5% ±9.2%, and declined in serum over time by -11.6% ±6.6% (at 12h). In the patient samples, serum TRAb concentration decreased over time by -4.6% ± 2.5% at day two and -6.5% ± 4.0% at day five.

CONCLUSION

TRAb concentration decreases over time in both whole blood at room temperature and serum samples stored at 4-8°C. A single freeze-thaw cycle also has a statistically significant effect on TRAb concentration.