Laboratory Testing: A Patient Guide to Understanding Your Test, Sample Collection, Results, and Safety

This test is a laboratory analysis performed on a sample of your blood, urine, or other body fluid to help your doctor evaluate your overall health, screen for certain medical conditions, monitor an ongoing illness, or check how well a current treatment is working for you.

This test looks for IgM antibodies to Borrelia burgdorferi, the bacterium that causes Lyme disease, in the synovial fluid of a joint. Synovial fluid is the slippery fluid that cushions and lubricates your joints. When Lyme disease spreads to the joints, it can cause inflammation and swelling, often in the knees or other large joints. This test helps doctors determine if Lyme disease bacteria are causing joint problems.

A trained healthcare professional will collect your sample in a clean, sterile setting. For a blood test, a small needle is used to draw blood from a vein in your arm, which usually takes only a few minutes. Some tests may instead require a urine sample, a swab, or another simple specimen that you may be able to provide at home.

A healthcare provider will first clean the skin over the affected joint with an antiseptic. They will then use a needle and syringe to withdraw a small amount of fluid from the joint (this procedure is called joint aspiration or arthrocentesis). The area may be numbed with a local anesthetic to reduce discomfort. The fluid sample is then sent to a laboratory for testing.

Your results are compared with established reference ranges to show whether your values fall within the expected normal range. An abnormal result does not always mean you have a serious condition, since factors such as medications, diet, hydration, and recent physical activity can affect your values. Your doctor will review the full report with you and explain what the findings mean for your ongoing care and any follow-up testing that may be needed.

A positive IgM result means your immune system has produced antibodies against the Lyme disease bacteria, suggesting a recent or active infection in the joint. A negative result could mean you do not have Lyme disease affecting the joint, or the infection is in an early stage where antibodies have not yet developed. Your doctor will interpret the results along with your symptoms, physical findings, and possibly other test results.

This type of testing is very safe, and serious complications are rare. You may feel a brief sting when the needle is inserted, and mild bruising, soreness, or slight bleeding at the collection site can occur afterward, usually resolving within a day or two. Let your provider know before the test if you have a bleeding disorder or take blood thinners, and report any unusual swelling, pain, or persistent bleeding afterward.

Joint aspiration is generally safe but may cause temporary pain, bruising, or swelling at the needle site. In rare cases, infection or bleeding may occur. Your healthcare provider will discuss the risks and benefits of the procedure with you.