Background and aims: Iron deficiency (ID) is probably the single most common nutrient deficiency in the world. It affects especially elderly persons, menstruating women, adolescent girls and infants. The concentration of the soluble transferrin receptor (sTfR) in the plasma is a relatively new laboratory test to detect ID. The aims of this study were to produce reference intervals (RIs) for sTfR and other hematological laboratory tests, to examine the influence of subclinical ID on the RIs and to establish the appropriate cut-off values for sTfR with regard to subclinical ID in elderly people, adolescent girls and preterm and full-term infants. A further objective was to establish the physiological changes of the hematological laboratory values during the first year of life among preterm and full-term infants. 

Results and conclusions: In elderly people and adolescent girls, the upper reference limit of sTfR declined when the RIs were based on the values of an iron-replete reference group compared to a conventional reference group. This indicates that the reference individuals, selected conventionally, may have had subclinical ID which influenced the RIs of sTfR and other markers of iron status. In infants, the RIs were the same when based on the iron-replete reference group as the whole reference group. This suggests that ID was not a marked problem for infants of this study. The iron-replete RIs for sTfR were 1.0–2.4 mg/L and 0.9–2.4 mg/L for elderly people and adolescent girls, respectively. In preterm infants, the RIs of sTfR increased from 1.1–2.2 mg/L to 1.2–2.4 mg/L with advancing age from 20 weeks to 60 weeks. The upper reference limits of the RIs of sTfR may thus be used as decision limits for ID. The values for the preterm and full-term infants provide also valuable information about the kinetics of the hematological laboratory values during the first year of life and may help physicians to interpret the results of these markers for clinical decision-making.