Complete Blood Count (CBC): What This Common Blood Test Measures and What Your Results Mean

What is this test? A complete blood count (CBC) is a routine blood test that measures the cells circulating in your blood, including red blood cells, white blood cells, and platelets. Doctors use it to evaluate your overall health and to screen for a wide range of conditions, such as infections, anemia, and blood disorders.

The Borrelia burgdorferi IgM Immunoblot test is a specialized blood test that detects IgM (immunoglobulin M) antibodies against the bacteria Borrelia burgdorferi, which causes Lyme disease. The immunoblot (also called Western blot) technique is highly specific and can identify antibodies to specific proteins of the Lyme bacteria, helping confirm an early infection.

How is the test performed? A healthcare professional will draw a small sample of blood, usually from a vein in your arm, using a thin needle. The procedure takes only a few minutes, and you may feel a brief pinch. A small bandage will be placed on the site afterward. No special preparation is usually required, although your doctor may give you specific instructions.

A healthcare professional will draw a blood sample from a vein in your arm. The blood is then sent to a laboratory where technicians separate the serum (the liquid part of blood) and test it using the immunoblot technique. This method detects antibodies that bind to specific bacterial proteins, creating visible bands that indicate a positive reaction to particular Lyme disease antigens.

What do the results mean? Your results show the number, size, and characteristics of your blood cells. Values that are higher or lower than the normal range can point to conditions such as infection, inflammation, anemia, dehydration, or clotting problems. Your doctor will interpret the numbers alongside your symptoms, medical history, and other tests, because a single abnormal value does not always indicate illness.

IgM antibodies usually appear first when the body fights a new infection. A positive or borderline IgM result suggests an early Lyme disease infection, particularly if you have symptoms like the characteristic bull's-eye rash (erythema migrans), fever, headache, or fatigue. However, the test results should always be interpreted in context with your symptoms, potential tick exposure, and timing of the test relative to when you were bitten.

What are the limitations of this test? A CBC provides broad information but cannot pinpoint a specific diagnosis on its own. Many factors, including recent illness, medications, hydration status, and normal day-to-day variation in your body, can influence the numbers. Abnormal results typically call for follow-up testing to identify the underlying cause.

In the early stages of infection (first 2-4 weeks), the test may be negative because the body hasn't had enough time to produce detectable antibodies. False positives can occur in some autoimmune conditions or with other infections. For this reason, the CDC recommends a two-tiered testing approach: if the initial ELISA screen is positive or equivocal, then the immunoblot is performed. A positive IgM result is considered significant only in the first 4-6 weeks of symptom onset.