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Unsaturated Iron-Binding Capacity (UIBC), also termed latent iron-binding capacity, measures the unoccupied iron-binding sites on transferrin and complements serum iron, total iron-binding capacity (TIBC), and transferrin for calculating transferrin saturation. This profile aids interpretation of iron transport and availability. UIBC supports evaluation of suspected iron deficiency or iron overload (including hereditary hemochromatosis), helps distinguish iron deficiency anemia from other causes of anemia such as anemia of chronic disease or vitamin B12 deficiency, and is used to monitor response to treatment for iron deficiency or excess.

Iron is an essential trace element incorporated into hemoglobin within erythrocytes, enabling oxygen transport, and is also present in myoglobin and multiple enzymes. Dietary iron is absorbed and transported in plasma by transferrin, a liver-derived protein. Total body iron averages 4–5 g, of which approximately 3–4 mg circulates bound to transferrin. Transferrin concentration reflects hepatic synthetic function and nutritional status, and under normal conditions roughly one-third of its binding sites are occupied by iron. Latent (unsaturated) iron-binding capacity represents the fraction of transferrin binding capacity not occupied by iron and is calculated as UIBC = TIBC − serum iron. In iron deficiency, transferrin production typically increases, leaving more unfilled binding sites and raising the UIBC. Conversely, in iron overload, most binding sites are occupied, and UIBC decreases. Serum iron shows notable diurnal and day-to-day variation, particularly in morning samples, whereas TIBC and UIBC are comparatively stable in healthy individuals. Early iron deficiency can be clinically silent; symptomatic anemia often becomes evident when hemoglobin declines below 100 g/L, with common complaints of fatigue, weakness, dizziness, and headaches.