Complete Blood Count (CBC) Test: What It Is, Why It Is Done, and What Your Results Mean

The complete blood count, or CBC, is a common blood test that measures the cells circulating in your blood, including red blood cells, white blood cells, and platelets. It gives your healthcare provider a broad picture of your overall health and helps detect conditions such as anemia, infections, and other blood-related disorders, often before you notice any symptoms.

This test looks for IgG (immunoglobulin G) antibodies specifically targeting the 23kD protein of Borrelia burgdorferi, the bacterium that causes Lyme disease. The test is performed on synovial (joint) fluid using a technique called immunoblot (western blot), which can detect even small amounts of specific antibodies. The 23kD protein is one of the key antigens used to confirm Lyme disease infection.

Your doctor may order a CBC as part of a routine physical exam, to help diagnose symptoms like fatigue, weakness, fever, bruising, or unexplained bleeding, or to monitor a condition you already have and how well your treatment is working. It is also frequently done before surgery to confirm that your blood counts are within a safe range.

This test is primarily ordered when a patient has symptoms of Lyme disease affecting the joints, such as swelling, pain, or stiffness in one or more joints (especially the knee). It helps confirm whether the joint symptoms are caused by a Borrelia burgdorferi infection. The test may be used when initial Lyme disease screening tests are inconclusive or to further characterize an immune response to the infection.

A small sample of blood is drawn from a vein, usually on the inside of your elbow or the back of your hand. A trained technician cleans the area, places a band around your upper arm, and inserts a thin needle to collect the blood into a tube. The process takes only a few minutes, and a small bandage is applied afterward.

Synovial fluid is collected from the affected joint using a procedure called arthrocentesis (joint aspiration). A healthcare provider inserts a thin needle into the joint space to withdraw fluid. The area will be cleaned and numbed with local anesthetic to minimize discomfort. The collected fluid is then sent to a laboratory for analysis.

Your results show whether each type of blood cell falls within the normal reference range. High or low values can suggest conditions such as anemia, infection, inflammation, dehydration, or clotting problems, but a single abnormal value does not confirm a diagnosis. Your provider will interpret the numbers together with your symptoms, medical history, and other test results before recommending any next steps.

A positive result indicates that IgG antibodies against the 23kD protein of Borrelia burgdorferi were detected in the joint fluid. This suggests that the patient's immune system has mounted a specific response to the Lyme disease bacteria, and the joint symptoms may be due to Lyme arthritis. A negative result does not completely rule out Lyme disease, as antibodies may not yet be present or may have declined over time. Results should always be interpreted in the context of clinical symptoms, exposure history, and other laboratory findings.

This test is very safe and complications are rare. You may feel a quick sting when the needle is inserted, and the puncture site may be slightly sore or bruised for a day or two. Let your provider know if you have a bleeding disorder or take blood thinners, since you may bleed a little more than usual at the site.

The main risks associated with joint fluid collection include slight bleeding, bruising, or infection at the puncture site. There is also a small risk of temporary joint discomfort or swelling after the procedure. These risks are minimal when performed by an experienced healthcare provider using proper sterile techniques.