Complete Blood Count (CBC)

The complete blood count, or CBC, is a common blood test that measures the different cells in your blood, including red blood cells, white blood cells, and platelets. These cells carry oxygen, fight infection, and help your blood clot, so this test gives your care team a broad picture of your overall health.

This test looks for antibodies in your blood that your immune system makes in response to Borrelia burgdorferi, the bacteria that causes Lyme disease. The test detects two types of antibodies: IgM (immunoglobulin M), which appear early in infection, and IgG (immunoglobulin G), which develop later. If the initial test is positive or unclear, additional tests may be performed to confirm the results.

This test is done for many reasons. It can help diagnose conditions such as anemia, infection, and certain blood disorders, monitor an ongoing illness or how a treatment is working, or simply be part of a routine checkup to make sure your body is working the way it should.

Your healthcare provider may order this test if you have symptoms of Lyme disease, especially if you remember being bitten by a tick or have been in areas where Lyme disease is common. Early symptoms include a characteristic bull's-eye rash (erythema migrans), fever, headache, and fatigue. If left untreated, the infection can spread to joints, the heart, and the nervous system.

In most cases, very little preparation is needed. You can usually eat and drink normally unless your doctor tells you otherwise, especially if the CBC is being done along with other tests that require fasting. Be sure to tell your care team about any medicines, vitamins, or supplements you take.

No special preparation is needed for this blood test. You can eat and drink normally before the test. Simply inform your healthcare provider about any medications or supplements you are taking, as they may affect test results in rare cases.

During the test, a healthcare professional will clean a small area of your skin, usually on the inside of your elbow or the back of your hand, and draw a small sample of blood into a tube. The procedure takes only a few minutes, and you may feel a quick sting or slight pinch when the needle goes in.

A healthcare professional will draw a small blood sample from a vein in your arm using a needle. You may feel a brief sting or pinch. The sample is then sent to a laboratory for analysis. Results are typically available within a few days.

Your results will show whether your blood cell counts fall within the normal reference ranges. Values that are too high or too low can point to conditions such as anemia, infection, inflammation, or clotting problems, but a single result is not enough for a diagnosis. Your doctor will review the numbers together with your symptoms, medical history, and any other tests before explaining what they mean for you.

Results are reported as positive, negative, or equivocal (unclear). A positive IgM result suggests early infection, while a positive IgG result indicates later or past infection. However, it takes several weeks for antibodies to develop, so testing too early after a tick bite may give a false negative. Your healthcare provider will interpret your results based on your symptoms and exposure history.