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Determination of the number of reticulocytes in a measured volume of blood. Values for reticulocytes are expressed as a percentage of the erythrocyte count or in the form of a so-called "corrected" reticulocyte "index". An increase in circulating reticulocytes, often referred to as reticulocytosis, is among the simplest and most reliable signs of accelerated erythrocyte production. Reticulocytosis, or an increased reticulocyte count, occurs during active blood regeneration (stimulation of red bone marrow) and in certain anemias, particularly congenital hemolytic anemia.