Complete Blood Count (CBC): What This Common Blood Test Measures and Why Your Doctor May Order It

The complete blood count, or CBC, is a routine blood test that measures the different cells circulating in your blood, including red blood cells, white blood cells, and platelets, giving your doctor a broad picture of your overall health.

This test, called Borrelia burgdorferi IgG and IgM by Immunoassay, is a blood test that checks for antibodies against the bacteria Borrelia burgdorferi, which causes Lyme disease. The test detects two types of antibodies: IgM (immunoglobulin M) and IgG (immunoglobulin G). IgM antibodies are the first ones your body makes when fighting a new infection, while IgG antibodies develop later and provide long-term immunity.

Your doctor may order this test as part of a routine checkup, to investigate symptoms such as fatigue, weakness, fever, or unusual bruising, to monitor a known condition like anemia or infection, or to track how a treatment such as chemotherapy is affecting your blood cells.

Your healthcare provider may order this test if you have symptoms of Lyme disease, especially if you recall being bitten by a tick or have been in areas where Lyme disease is common. Early symptoms may include a bull's-eye rash (erythema migrans), fever, headache, and fatigue. Later symptoms can include joint pain, neurological problems, and heart issues. The test helps determine if your symptoms are due to a Lyme disease infection.

A healthcare professional will wrap an elastic band around your upper arm, clean the skin at the inside of your elbow, and draw a small sample of blood into a tube using a thin needle. The sample is then sent to a laboratory, and the collection itself usually takes less than five minutes.

A healthcare professional will draw a small blood sample from a vein in your arm using a needle. The blood sample is then sent to a laboratory where technicians use special tests (immunoassays) to detect the presence and amount of IgM and IgG antibodies against Borrelia burgdorferi. The entire process is similar to routine blood tests and takes only a few minutes to collect the sample.

Your report will list each blood component alongside a reference range, which shows the values typically considered normal for healthy adults. Values slightly outside these ranges do not always indicate a problem, so your doctor will interpret the numbers together with your symptoms, medical history, and other results before recommending any next steps.

Results are typically reported as positive, negative, or equivocal for both IgM and IgG antibodies. A positive IgM result with a negative IgG result may indicate an early infection. A positive IgG result (with or without IgM) typically indicates a later or past infection. However, it takes time for antibodies to develop—testing too early (within 2-4 weeks of the tick bite) may give a false-negative result even if you are infected. Your healthcare provider will interpret your results together with your symptoms, medical history, and potential exposure to ticks.

This test is very safe. You may feel a quick sting when the needle is inserted, and the puncture site can be slightly sore or bruised afterward. Some people feel lightheaded during a blood draw, but serious complications are extremely rare, and most people return to their normal activities immediately.

The risks associated with this blood test are minimal and rare. You may feel brief pain or bruising at the needle insertion site. In very rare cases, excessive bleeding, infection, or fainting may occur. These risks are the same as with any standard blood draw.