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What is this test?

The Borrelia burgdorferi 41kD IgM Antibody test is a specialized laboratory test that looks for IgM (immunoglobulin M) antibodies in the synovial (joint) fluid. IgM antibodies are the first type of antibodies your body produces when fighting a new infection. The test uses a technique called immunoblot (also known as Western blot) to detect antibodies specifically targeting the 41kD protein of Borrelia burgdorferi, the bacterium that causes Lyme disease.

Why is this test performed?

This test is primarily ordered when a patient has symptoms of Lyme disease affecting the joints, such as swelling, pain, or stiffness, especially in the knee or other large joints. It helps doctors confirm whether the joint symptoms are caused by Lyme disease bacteria. The test is particularly useful when initial screening tests like ELISA are positive or equivocal, and confirmation is needed.

What happens during the test?

A healthcare provider will first extract a small sample of synovial fluid from the affected joint using a needle and syringe (joint aspiration). This procedure is called arthrocentesis. The fluid sample is then sent to a laboratory where technicians will perform the immunoblot test to detect the presence and quantity of IgM antibodies against the 41kD protein of Borrelia burgdorferi.

What do the results mean?

A positive result indicates that IgM antibodies against Borrelia burgdorferi were detected in the joint fluid, suggesting that the patient has or recently had a Lyme disease infection affecting the joint. A negative result means no IgM antibodies were detected, but this does not completely rule out Lyme disease, especially if the infection is very early or in a later stage. Your doctor will interpret these results together with your symptoms, medical history, and other test results.