Anemia is defined as a condition with a reduced total weight of red blood cells in the bloodstream. It is called sideropenic anemia when the cause is absolute or functional iron deficiency. There are many different laboratory tests that evaluate the levels and types of anemia. Diagnostic indicators in basic laboratory findings are decreased hemoglobin and hematocrit, hypochromic microcytic erythrocytes and the presence of poikilocytes in the peripheral blood smear. The most commonly used methods to determine the storage pool of iron are serum ferritin and saturation of transferrin with iron. Determining the level of iron with these indicators is sufficient in otherwise healthy individuals, however, it becomes problematic in patients with inflammatory conditions or chronic disease, when treating human recombinant erythropoietin, the elderly or children. In these groups, standard indicators are too variable, which is why the concentration of hemoglobin in reticulocytes and the percentage of hypochromic erythrocytes are more precise indicators. The concentration of hemoglobin in reticulocytes is a very sensitive and early indicator of functional iron deficiency. An increased percentage of hypochromic erythrocytes in the bloodstream shows a long-term absolute or functional iron deficiency. Its advantage is the detection of iron deficiency in dialysis patients and functional iron deficiency in patients treated with human recombinant erythropoietin. To manage sideropenic anemia, iron replacement and the cause of the deficit have to be treated simultaneously. Iron can be replaced with oral therapy, intravenous application and optimization of dietary intake (especially as secondary prevention).