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.
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.
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 due largely to variance in the dilution of the sample and the distribution of cells in the chamber, as well as the small number of WBCs that are counted. For electronic WBC counts and differentials, interference may be caused by small fibrin clots, nucleated red blood cells (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 WBCs in other body fluids, especially when the number of WBCs is less than 1000/μL or when other nucleated cell types are present.
White cell counts are usually performed using an automated instrument, but may be done manually using a microscope and a counting chamber, especially when counts are very low, or if the patient has a condition known to interfere with an automated WBC count.
An automated differential may be performed by an electronic cell counter or by an image analysis instrument. When the electronic WBC count is abnormal or a cell population is flagged, meaning that one or more of the results is atypical, a manual differential is performed. The WBC differential is performed manually by microscopic examination of a blood sample that is spread in a thin film on a glass slide. White blood cells are identified by their size, shape, and texture.
The manual WBC differential involves a thorough evaluation of a stained blood film. In addition to determining the percentage of each mature white blood cell, the following tests are preformed as part of the differential:
This test requires a 3.5 mL sample of blood. Vein puncture with a needle is usually performed by a nurse or phlebotomist, a person trained to draw blood. There is no restriction on diet or physical activity.
Discomfort or bruising may occur at the puncture site. Pressure to the puncture site until the bleeding stops reduces bruising; warm packs relieve discomfort. Some people feel dizzy or faint after blood has been drawn and should be allowed to lie down and relax until they are stable.
Other than potential bruising at the puncture site, and/or dizziness, there are no complications associated with this test.
Normal values vary with age. White blood cell counts are highest in children under one year of age and then decrease somewhat until adulthood. The increase is largely in the lymphocyte population. Adult normal values are shown below.
Chernecky, Cynthia C., and Barbara J. Berger. Laboratory Tests and Diagnostic Procedures, 3rd ed. Philadelphia, PA: W. B. Saunders Company, 2001.
Henry, J. B. Clinical Diagnosis and Management by Laboratory Methods, 20th ed. Philadelphia, PA: W.B. Saunders Company, 2001.
Kee, Joyce LeFever. Handbook of Laboratory and Diagnostic Tests, 4th ed. Upper Saddle River, NJ: Prentice Hall, 2001.
Wallach, Jacques. Interpretation of Diagnostic Tests, 7th ed. Philadelphia, PA: Lippincott Williams & Wilkens, 2000.
National Institutes of Health. [cited April 5, 2003] http://www.nlm.nih.gov/medlineplus/encyclopedia.html .
Victoria E. DeMoranville Mark A. Best