Understanding Your Medical Test: A Complete Patient Guide

This test is a laboratory procedure that helps your healthcare provider learn how your body is functioning. It measures specific substances in a sample taken from your body, giving your care team objective information that cannot be obtained from a physical exam or your medical history alone.

This test detects IgG (immunoglobulin G) antibodies specifically targeting the 21kD protein of Borrelia burgdorferi, the bacterium that causes Lyme disease. The test is performed on synovial fluid, which is the fluid that lubricates joints. By using an immunoblot (also known as Western blot) technique, the test can identify whether your immune system has produced specific antibodies against this particular bacterial protein.

Your doctor may order this test to look for signs of illness, confirm or rule out a suspected diagnosis, monitor a condition you already have, or check how well a treatment is working. It can also be part of a routine checkup or screening to catch potential problems early, before symptoms appear.

This test is primarily ordered when a patient has symptoms suggesting Lyme disease involvement in the joints, such as joint swelling, pain, or stiffness—particularly in the knees, though other joints may be affected. It helps healthcare providers confirm whether the joint symptoms are related to Lyme disease infection. The 21kD protein is one of the specific markers used in Lyme disease testing to ensure accuracy.

The test is usually done by collecting a small sample, most often blood drawn from a vein in your arm or sometimes urine or another specimen. A trained healthcare professional will clean the area, collect the sample, and send it to a laboratory for analysis. The procedure typically takes only a few minutes, and you can usually return to your normal activities right afterward.

A healthcare provider will collect a sample of synovial fluid from the affected joint using a needle and syringe in a procedure called arthrocentesis. This fluid is then sent to a laboratory where it is tested using the immunoblot technique. This method separates the bacterial proteins by size and detects whether IgG antibodies against the 21kD protein are present.

Your results show whether your measurements fall within the normal reference range for someone of your age and sex. Results within the normal range generally suggest no evidence of the condition being tested for, while results outside that range may point to a condition that needs further evaluation. Abnormal results do not always mean something serious, so your healthcare provider will interpret them together with your symptoms, history, and other tests before recommending next steps.

A positive result indicates that IgG antibodies against the Borrelia burgdorferi 21kD protein were detected in your synovial fluid. This suggests your immune system has been exposed to the Lyme disease bacterium and has produced an antibody response. A negative result means no antibodies against this specific protein were detected, though it does not completely rule out Lyme disease—especially if the infection is very recent. Your healthcare provider will interpret these results together with your symptoms, medical history, and other test findings.