(1) Background: The serum ferritin cut-off to define absolute iron deficiency is not well-established. The aim of the present study was to determine a clinically relevant ferritin threshold by using early serum biomarkers of iron deficiency such as hepcidin and the soluble transferrin receptor; (2) Methods: Two hundred and twenty-eight asymptomatic subjects attending a hospital as outpatients between 1st April 2020 and 27th February 2022 were selected. Iron metabolism parameters as part of the blood analysis were requested by their doctor and included in the study. Then, they were classified into groups according to their ferritin levels and iron-related biomarkers in serum were determined, quantified, and compared between ferritin score groups and anemic subjects. (3) Results: Serum ferritin levels below 50 ng/mL establish the point from which the serum biomarker, the soluble transferrin receptor to hepcidin ratio (sTfR/Hep ratio), begins to correlate significantly with ferritin levels. (4) Conclusion: Ferritin levels ≤ 50 ng/mL are indicative of early iron deficiency; hence, this should be considered as a clinically relevant cut-off for iron deficiency.
Keywords: ferritin; hepcidin; iron deficiency; soluble transferrin receptor.