A complete blood count (CBC) is a routine blood test that measures the composition of blood cells within the body. The test provides physicians with important information about the white blood cells (WBCs), red blood cells (RBCs) and platelets in a person’s blood. This information includes the number, type, size, shape and some of the other physical characteristics of the cells.

The results of a CBC can help diagnose various conditions and diseases, including some that cause chronic pain, such as rheumatoid arthritis.

High WBC levels may indicate an inflammatory disease, infection, heart attack or cancer. Low levels can be caused by kidney or liver disease, which are common complications of some rheumatic conditions, including lupus. Infection can also cause a low WBC level, especially in the elderly.

High RBC levels may indicate dehydration, whereas low levels indicate anemia.

High platelet levels may be due to severe bleeding, infection, inflammation, strenuous exercise, pregnancy or other factors. Low levels are usually caused by an infection, cancer or a lack of folic acid or vitamin B12.

Some medications may interfere with a CBC. Patients are urged to follow their physician’s orders carefully about how to take their medications before the test in order to ensure its accuracy. There is no need to stop eating or drinking beforehand.

After a CBC, additional diagnostic tests such as urinalysis, imaging tests or biopsy may be necessary to confirm a diagnosis. Additionally, patients may need frequent CBC testing to monitor their condition while it is being treated. Patients may also require periodic testing while taking medications that affect the blood, such as nonsteroidal anti-inflammatory drugs (NSAIDs), which are commonly prescribed for conditions such as arthritis and back pain.