Objectives: Establishment of reference values by direct strategy for selection of reference individuals is time-consuming and expensive for laboratories. The aim of this study was to establish reference intervals for thyrotrophin(TSH), free thyroxine (FT4), free triiodothyronine (FT3) in a middle-aged and elderly population (age range; 40-80 years) on an immunoassay analyzer using both direct and indirect strategies, and to compare the results of direct and indirect strategies.
Materials and methods: As for direct sampling strategy, healthy reference individuals were randomly selected (129 male, 131 female; age range: 40-80) by questionnaire. As for indirect sampling strategy, the results of TSH, FT3, FT4 measurements of all individuals (male, n=3060-3920; female, n=4090-4520; age range 40-80 years), were selected from laboratory information system. Reference intervals were calculated by nonparametric methods
Results: The following 95% reference intervals were determined: For direct sampling group: TSH 0.32-4.38 µIU/mL; FT4 0.75-1.32 ng/dL; FT3 1.98-3.65 pg/mL. For indirect sampling group: TSH 0.29-4.36 µIU/mL; FT4 0.71-1.37 ng/dL; FT3 1.69-3.32 pg/mL. Indirect sampling group showed a good concordance in the reference intervals for TSH and FT4 with direct sampling group.
Conclusion: Indirect sampling strategy and nonparametric method may be confidently used to evaluate the reference intervals of serum TSH and FT4 on an analyzer. We suggested the utilization of a very simple and practical way, indirect sampling strategy, for evaluating reference intervals for elderly population.