Why get tested?
To diagnose an illness affecting your immune system, such as an infection or viral condition, or other diseases that affect your white blood cells, such as leukemias

When to get tested?
As part of a complete blood count (CBC), which may be ordered for a variety of reasons

Sample required?
A blood sample drawn from a vein in your arm or by a fingerstick (children and adults) or heelstick (infants)