Determination of reference change values for thyroid-related biomarkers: TSH, fT3, fT4, Tg, Anti-Tg, and Anti-TPO

Abstract Objectives Reference change value (RCV) is an objective tool that indicates whether the change between consecutive measurements is clinically significant. This study aimed to determine the instrument-specific RCVs of thyroid-stimulating hormone (TSH), free triiodothyronine (fT3), free thyroxine (fT4), thyroglobulin (Tg), thyroglobulin antibody (Anti-Tg), and thyroperoxidase antibody (Anti-TPO) tests, which are routinely analyzed for the diagnosis, case-finding, and screening of thyroid diseases. Methods This cross-sectional study was conducted between January 1 and December 31, 2018. The twelve-month mean analytical imprecision (CV A ) was calculated from internal quality control data for each test on three different instruments. All tests’ individuality index (II) was calculated using the biological variation data. RCVs were determined by both conventional and logarithmic approaches. Results It was shown that the II of all tests was 0.6 and below. Upward logarithmic RCVs were slightly higher than conventional RCVs, while downward logarithmic RCVs were lower (e.g., conventional RCV for TSH 39 %, whereas upward and downward RCVs 48 and 32 %, respectively). Conclusions Due to high individuality, the results of thyroid-related biomarkers would be more appropriate to evaluate with RCV instead of reference interval. Instrument-specific RCVs should be determined because the analytical imprecision of analyzers may vary. Most presentations to the hospital are not healthy individuals, so logarithmic RCV could provide a more accurate assessment of significant change.