Understanding Your Laboratory Test: A Patient Guide

A laboratory test is a medical procedure in which a sample of your blood, urine, or other body fluid or tissue is analyzed to help your healthcare provider evaluate your overall health, detect a possible condition, or keep track of an illness you already have.

This test looks for IgG antibodies specifically targeting the 18kD protein of Borrelia burgdorferi in your synovial (joint) fluid. The immunoblot method is highly specific and can distinguish true infections from false positives. When the immune system detects Borrelia burgdorferi, it produces specific antibodies; this test identifies whether your joint fluid contains these specific antibodies.

Your provider may order this test to find out how well your body is working, look for signs of disease, confirm or rule out a suspected diagnosis, screen for certain conditions before symptoms appear, or check whether a treatment you are receiving is working as expected.

This test is typically ordered when a patient has symptoms of Lyme disease affecting the joints, such as swelling, pain, or stiffness, especially in large joints like the knee. It helps confirm whether Lyme disease bacteria have spread to the joint, which can occur in early disseminated or late-stage Lyme disease.

How your sample is collected depends on the type of test being done. It may involve drawing blood from a vein in your arm, asking you to provide a urine sample, or taking a small swab from a specific area of your body. Collection is usually quick and is performed by trained staff, and you can typically return to your normal activities right afterward.

A healthcare provider will perform a joint aspiration (arthrocentesis) to collect synovial fluid from the affected joint. The area is first cleaned and numbed with local anesthetic. A needle is then inserted into the joint space to withdraw fluid. The procedure takes only a few minutes, and you may feel pressure or mild discomfort.

Your results are compared with a reference range, which is the set of values seen in most healthy people. Results that fall within this range are generally considered normal, while values outside it may point to a condition that needs further attention. Your healthcare provider will explain what your specific results mean for you and will discuss any follow-up testing, treatment options, or lifestyle changes that may be helpful.

Results are reported as positive or negative for the 18kD IgG antibody. A positive result indicates that IgG antibodies against the 18kD protein of Borrelia burgdorferi were detected in your joint fluid, suggesting an immune response to Lyme disease bacteria in the joint. A negative result means no antibodies were detected, but it does not completely rule out Lyme disease if the infection is very early or in some chronic cases.