TF and heart failure: Due to a lack of international consensus, iron deficiency is often defined as (1) serum ferritin < 100 ng/mL, or 100–299 ng/mL with transferrin saturation <20%, which is the guideline definition for heart failure patients, (2) serum iron concentration ≤ 13 μmol/L, or (3) transferrin saturation < 20% [193].