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Published on: October 6, 2023
Indirect reference intervals for thyroid function tests: refineR, RefLim and effects of pathological test results
Finn Erik Aas1, Alexander Bauer Westbye1, Magnus A R Husøy2
1Hormone Laboratory, Dept. Medical Biochemistry, Oslo University Hospital, Aker, PO Box 4959 Nydalen, 0424 Oslo, Norway; Endocrinology and Metabolism Research Group, Oslo University Hospital, Aker, PO Box 4959 Nydalen, 0424 Oslo, Norway.
Background:
Biochemical tests with reference intervals (RIs) are extensively used in medicine. RIs should reflect the population tested, and indirect methods for RI construction are increasingly used. Thyroid function tests (TFTs) are used to diagnose and monitor thyroid disorders, that are especially common among women. We report investigations into factors that influence indirect TFT RIs using real world data.
Methods:
Indirect RIs were calculated (refineR and RefLim) for two laboratories with very different proportions of pathological results for thyroid stimulating hormone (TSH), free thyroxine (FT4) and free triiodothyronine (FT3), all measured on Abbott Alinity i. For one laboratory total thyroxine (TT4) and total triiodothyronine (TT3) were also investigated. Effects of pathological results on indirect RIs were investigated and indirect RIs were compared to direct RIs from Abbott and from a reference population (HUNT4).
Results:
Some TFT RIs differed markedly between the two laboratories, but removal of suspected pathological results reduced the discrepancy. Choice of algorithm for indirect RIs influenced the RIs, and indirect RIs were wider than the direct RIs from HUNT4. Most of our calculated RIs - from indirect (real world data) or direct (HUNT4) methods - were markedly narrower than Abbott-provided RIs, particularly for FT4 and FT3.
Conclusions:
Indirect RIs for TFT are easily calculated but are affected by several factors that laboratories should be aware of. Manufacturer-provided RIs can differ markedly from in-house RIs.
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