What laboratory finding is characteristic for hemolytic anemias?

Elevated reticulocyte count is a hallmark laboratory finding in hemolytic anemias. When red blood cells are destroyed at an accelerated rate, the bone marrow responds by increasing the production of new red blood cells to compensate for the loss. Reticulocytes are immature red blood cells that are released into the bloodstream and indicate active erythropoiesis. In hemolytic anemias, the increased destruction of red blood cells leads to a higher than normal reticulocyte count as the bone marrow attempts to recover from the loss and meet the body's oxygen demands.

In contrast, increases in vitamin B12 levels, decreased ferritin levels, and elevated mean corpuscular volume are not characteristic of hemolytic anemia. Vitamin B12 is typically associated with megaloblastic anemias; decreased ferritin levels indicate iron deficiency; and a high mean corpuscular volume is associated with various forms of macrocytic anemia rather than hemolytic processes. Thus, the elevated reticulocyte count specifically signifies the bone marrow's reaction to hemolysis, making it the most relevant finding in this context.