Background: Systematic difference between thyroid-stimulating hormone (TSH) immunoassays may produce misleading interpretation when samples of the same patients are measured with different methods. The study aims were to evaluate whether systematic differences are present among TSH immunoassays, and whether it is possible to obtain a better harmonization among TSH methods using results obtained in external quality assessment (EQA) schemes.
Methods: Seven Italian clinical laboratories measured TSH in 745 serum samples of healthy subjects and patients with thyroid disorders. These samples were also re-measured by two reference laboratories of the study with the six TSH immunoassays most popular in Italy after 2 months of storage at -80 °C. Moreover, these data were compared to 53,823 TSH measurements, obtained by laboratories participant to 2012-2015 EQA annual cycles in 72 quality control samples (TSH concentrations from about 0.1 mIU/L to 18.0 mIU/L). TSH concentrations were recalibrated using a mathematical approach based on the principal component analysis (PCA).
Results: Systematic differences were found between the most popular commercially available TSH immunoassays. TSH concentrations measured by the clinical laboratories were very closely correlated to those measured with the same method by reference laboratories after 2 months of storage at -80 °C. After recalibration using the PCA approach the variation of TSH values significantly decreased from a median pre-calibration value of 13.53% (10.79%-16.53%) to 9.63% (6.90%-13.21%) after recalibration.
Conclusions: Our data suggest that EQA schemes are useful to improve harmonization among TSH immunoassays and also to produce some mathematical formulas, which can be used by clinicians to better compare TSH values measured with different methods.
Keywords: harmonization; immunoassay methods; quality control; quality specification; standardization; thyroid-stimulating hormone (TSH).