Understanding Your Laboratory Test: A Patient's Guide to Its Purpose, Sample Collection, Results, and Safety

This test is a laboratory analysis performed on a sample of your blood, urine, or another body fluid to help your healthcare provider evaluate how well your body is functioning. The measurements it produces can detect disease, monitor an ongoing condition, or confirm that a treatment is working as intended.

This test looks for IgG antibodies against the 30kD protein of Borrelia burgdorferi (the bacterium that causes Lyme disease) in your synovial fluid—the fluid that lubricates your joints. The immunoblot (also called Western blot) technique is used to detect these specific antibodies, which your immune system produces in response to a Lyme infection.

Your doctor may order this test as part of a routine checkup, to investigate symptoms you have been experiencing, to screen for a condition before it causes problems, or to track how a known illness is responding to therapy. It may also be used before surgery or when starting a new medication to establish a baseline.

Your healthcare provider may order this test if you have joint pain, swelling, or stiffness—especially in the knees—and Lyme disease is suspected. It is particularly useful when standard blood tests for Lyme disease are negative or inconclusive, but joint symptoms persist. This test helps determine if the Lyme bacteria have spread to your joints, causing what is known as Lyme arthritis.

A trained healthcare professional will collect the sample, most often by drawing a small amount of blood from a vein in your arm with a sterile needle. Depending on the test, you may instead be asked to provide a urine sample, a swab, or another specimen, and you will receive clear instructions on any preparation needed.

A healthcare provider will collect synovial fluid from your affected joint using a needle and syringe in a procedure called joint aspiration or arthrocentesis. The area will be cleaned and numbed to minimize discomfort. The fluid is then sent to a laboratory for analysis.

Your results are compared with a reference range, which is the interval of values typically seen in healthy people. Values within that range are generally considered normal, while results that are higher or lower may point to infection, inflammation, organ dysfunction, or another issue. Your provider will interpret the numbers alongside your symptoms, medical history, and other findings before making a diagnosis or recommending next steps.

A positive result indicates that IgG antibodies against the 30kD protein of Borrelia burgdorferi were detected in your synovial fluid. This suggests your immune system has responded to the Lyme bacteria and that the infection may be affecting your joint. A negative result means no antibodies were detected, but this does not completely rule out Lyme disease—it may be too early in the infection, or the bacteria may not have reached the joint. Always discuss your results with your healthcare provider who will consider your symptoms, medical history, and other test results.

This test is very safe. The most common effects are brief pain or bruising at the needle site and, occasionally, lightheadedness during the blood draw. Serious complications such as infection or excessive bleeding are extremely uncommon. If you feel dizzy after the collection, tell the staff right away, and apply gentle pressure to the site if you notice any swelling.

The main risks of joint fluid collection include temporary pain, bruising, swelling, or very rarely, infection at the puncture site. Your healthcare provider will take precautions to minimize these risks. The procedure is generally well-tolerated and takes only a few minutes.