White Blood Cell Count and Differe... Health Article

Definition

A white blood cell (WBC) count determines the concentration of white blood cells in the patient's blood. A differential determines the percentage of each of the five types of mature white blood cells.

Purpose

This test is included in general health examinations and to help investigate a variety of illnesses. An elevated WBC count occurs in infection, allergy, systemic illness, inflammation, tissue injury, and leukemia. A low WBC count may occur in some viral infections, immunodeficiency states, and bone marrow failure. The WBC count provides clues about certain illnesses, and helps physicians monitor a patient's recovery from others. Abnormal counts which return to normal indicate that the condition is improving, while counts that become more abnormal indicate that the condition is worsening. The differential will reveal which WBC types are affected most. For example, an elevated WBC count with an absolute increase in lymphocytes having an atypical appearance is most often caused by infectious mononucleosis. The differential will also identify early WBCs which may be reactive (e.g. a response to acute infection) or the result of a leukemia.

Precautions

Many medications affect the WBC count. Both prescription and non-prescription drugs including herbal supplements should be noted. Normal values for both the WBC count and differential are age related.

Sources of error in manual WBC counting are largely due to variance in the dilution of the sample and the distribution of cells in the chamber, and the small number of WBCs that are counted. For electronic WBC counts and differentials, interference may be caused by small fibrin clots, nucleated RBCs, platelet clumping, and unlysed RBCs. Immature WBCs and nucleated RBCs may cause interference with the automated differential count. Automated cell counters may not be acceptable for counting white blood cells in other body fluids especially when the number of WBCs is less than 1000/μL or when other nucleated cell types are present.