Anemia is defined as reduction in the oxygen carrying capacity of blood, as observed by reduced levels of hemoglobin concentration and red cell mass (Hematocrit) leading to tissue hypoxia. It reflects the disturbance of the dynamic balance between production and destruction of erythrocytes and hemoglobin. In normal subjects, the average life span of red cell i.e. time between the release of red cell from bone marrow and its disappearance from circulation, is between 100 to 120 days. The cells destroyed each day are replaced by new cells released from marrow, with the result the red cell population in the blood consists of cells ranging in the age from one to 120 days. Thus, approximately 1% or slightly less of body's red cells are destroyed and replaced each day. Any disruption of this balance - such as reduced production or increased destruction leads to anemia. The aged cells are removed from circulation by the reticuloendothelial cells, principally in the walls of sinusoids of reticuloendothelial system, where the flow of blood is slow, particularly in the splenic pulp. Child is said to be anemic when the hemoglobin and / or hematocrit is two standard deviation below mean for that particular age and sex.

The following table gives the normal values (normal mean and lower limit of normal) of various hematological parameters at different age groups (Table 1)

Table 1: AVERAGE NORMAL BLOOD VALUES AT DIFFERENT AGE GROUPS

• RBC size - micro, normo, and macrocytic anemia.
• RBC shape - e.g. Sickle cell.
• Etiology