Conditions or medications that weaken the immune system, such as HIV infection or chemotherapy, cause a decrease in white blood cells. The WBC count detects dangerously low numbers of these cells.

The WBC count is used to suggest the presence of an infection, an allergy, or leukemia. It is also used to help monitor the body’s response to various treatments and to monitor bone marrow function.

A WBC count is normally ordered as part of the complete blood count (CBC), which is ordered for a wide variety of reasons. A WBC count also may be ordered to monitor recovery from illness. Counts that continue to rise or fall to abnormal levels indicate that the condition is getting worse. Counts that return to normal indicate improvement.

An elevated number of white blood cells is called leukocytosis. This can result from bacterial infections, inflammation, leukemia, trauma, or stress. A WBC count of 11.0–17.0x109/L cells would be considered mild to moderate leukocytosis.

A decreased WBC count is called leukopenia. It can result from many different situations, such as chemotherapy, radiation therapy, or diseases of the immune system. A count of 3.0–5.0x109/L cells would be considered mild leukopenia.

In most cases, test results are reported as numerical values rather than as "high" or "low", "positive" or "negative", or "normal". In these instances, it is necessary to know the reference range for the particular test. However, reference ranges may vary by the patient's age, sex, as well as the instrumentation or kit used to perform the test. To learn more about reference ranges, please see the article, Reference Ranges and What They Mean. To learn the reference range for your test, consult your doctor or laboratorian.

Eating, physical activity, and stress can cause an increased WBC count.

Pregnancy in the final month and labor may be associated with increased WBC levels.

If you have had your spleen removed, you may have a persistent mild to moderate increased WBC count.

The WBC count tends to be lower in the morning and higher in the late afternoon. WBC counts are age-related.

On average, normal newborns and infants have higher WBC counts than adults. It is not uncommon for the elderly to fail to develop leukocytosis as a response to infection.

There are many drugs that cause both increased and decreased WBC counts.