It is known that apart from the WHO definitions of anaemia [31], a low TfS in fasting blood samples (<20%) and a serum ferritin concentration of less than 30 μg/L (with a serum CRP level within the normal range or a ferritin concentration of less than 100 μg/L with an elevated serum CRP level) are suitable laboratory tests for the diagnosis and assessment of iron deficiency in IBD used in the randomized, controlled studies [164]. Here, CRP is linked to anemia (phenotype).