Which laboratory finding is diagnostic of iron deficiency anemia?

Low mean corpuscular volume (MCV) is a diagnostic hallmark of iron deficiency anemia. In iron deficiency, there is insufficient iron available for hemoglobin synthesis, leading to the production of smaller, less hemoglobin-rich red blood cells. This reduction in size is reflected in a decreased MCV, which measures the average volume of a red blood cell. A low MCV indicates that the red blood cells are microcytic, which is characteristic of iron deficiency anemia.

In contrast, increased serum ferritin typically suggests adequate or high iron stores, which would not be consistent with iron deficiency anemia. Decreased total iron-binding capacity (TIBC) usually indicates anemia of chronic disease rather than iron deficiency since patients with iron deficiency tend to show increased TIBC as the body attempts to maximize iron utilization. Normal serum iron levels do not indicate iron deficiency anemia, as this condition is often associated with decreased serum iron. Therefore, the identification of a low MCV in the context of symptoms or other laboratory findings often guides the diagnosis of iron deficiency anemia.