When evaluating RBC morphology on a peripheral smear, the intensity of color and the size of the central pallor will reflect the amount of hemoglobin in the population of RBCs. RBCs that appear disc-shaped and have an area of central pallor that occupies approximately one-third of the cell's diameter contain the "normal" amount of hemoglobin and are considered "normochromic."
RBCs that appear disc-shaped and have an area of central pallor or lightening of color that is larger than expected are considered "hypochromic." Although hypochromic cells can vary from those with a slight increase in central pallor to cells with just a thin ring of hemoglobin, the term is used to describe the number of RBCs exhibiting increased central pallor rather than the increased central pallor of any individual cell. Hypochromia generally parallels the MCHC. As the MCHC decreases, the likelihood of hypochromia increases but, like the MCV, the MCHC is an average measurement and may not reflect a mixed population of cells. Hypochromia is most often associated with microcytosis, rarely seen in normocytic cells, and not seen in macrocytic disorders.
Lack of central pallor due to increased hemoglobin concentration in the red cell results in a dense, deep staining RBC called a spherocyte (two spherocytes are indicated by the arrows in the image on the right). These are the only cells that are truly hyperchromic, containing more hemoglobin than normal in relation to the cell volume. Hyperchromia parallels the MCHC. As the MCHC increases, the likelihood of spherocytes being present increases. However, the MCHC is prone to false elevation due to interfering substances such as lipemia, hemolysis, and red cell agglutination. Note that hyperchromia is generally not a reported descriptive term.
