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

Published: August 9, 2026Updated: August 16, 2026

This test detects specific antibodies against the 39kD protein of Borrelia burgdorferi bacteria in joint fluid to help diagnose Lyme disease arthritis.

Quick summary

{"Detects IgG antibodies to Borrelia burgdorferi (Lyme disease bacteria) in joint fluid","Uses immunoblot (Western blot) technique for high specificity","Helps confirm Lyme disease arthritis when patients have joint swelling","Sample is collected via joint aspiration (arthrocentesis)","Positive result indicates the immune system has responded to Lyme disease bacteria"}

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC: 42239-4
Short code: BB39IGGSF
Synonyms: B burgdor39kD IgG Snv Ql IB
Category: chemistry
Sample: fluid
Container: Sterile, screw-capped container; no anticoagulant
Units: N/A
Fasting: false
Population: adult
Sex: all
Age range: 18–65 years
Method: Immunoblot (Western Blot)
CPT: 86618
Specimen volume: 1-2 mL synovial fluid
Stability: Refrigerated at 2-8°C for up to 7 days; frozen at -20°C for longer storage
Hemolysis max: 300
TAT: 48–168 hours
Order: routine

What is this test and why might my healthcare provider recommend it for me?

This test looks for IgG antibodies specifically targeting the 39kD protein of Borrelia burgdorferi, the bacterium that causes Lyme disease. The test is performed on fluid extracted from an inflamed joint (synovial fluid). The immunoblot method separates bacterial proteins by size and detects if your immune system has produced antibodies against the 39kD protein, which is a key marker of Lyme disease infection.

How is the sample for this test collected, and what can I expect during the process?

A healthcare provider will perform a procedure called arthrocentesis (joint aspiration). After numbing the area, a thin needle is inserted into the joint to withdraw fluid. The procedure is similar to drawing blood but targets the joint space. You may feel pressure or mild discomfort during the procedure. The collected fluid is then sent to a laboratory for analysis.

What do the results of this test mean for my health and treatment plan?

A positive result means IgG antibodies against the Borrelia burgdorferi 39kD protein were detected in your joint fluid. This suggests your body has mounted an immune response to Lyme disease bacteria, supporting the diagnosis of Lyme disease arthritis. A negative result does not completely rule out Lyme disease, as antibodies may not yet be present in early stages or the infection may be caused by a different Borrelia strain.

Are there any risks, side effects, or complications I should know about?

Risks of joint aspiration include minor bleeding, bruising, or infection at the puncture site. Some patients may experience temporary joint discomfort or swelling after the procedure. These risks are generally minor and resolve within a few days.

Reference range

NegativeN/A

Specimen: fluid

No special preparation required. Arthrocentesis should be performed under sterile conditions by qualified personnel.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

This test is primarily used to confirm Lyme disease arthritis, one of the late manifestations of Lyme disease. When a patient presents with joint swelling (typically in the knee) and has a history of potential tick exposure, detecting 39kD IgG antibodies in synovial fluid provides strong evidence of Lyme disease as the cause. The 39kD protein is considered a specific antigen for Borrelia burgdorferi, making this test highly valuable for differential diagnosis of infectious versus inflammatory arthritis.

Clinical utility: This test helps guide appropriate antibiotic treatment for Lyme disease arthritis. It also aids in distinguishing Lyme arthritis from other conditions such as rheumatoid arthritis, psoriatic arthritis, or gout that can cause similar joint symptoms.

Frequently asked questions

Why test synovial fluid instead of blood?

Testing synovial fluid provides direct evidence of the immune response occurring within the joint itself. While blood tests can indicate systemic infection, detecting antibodies specifically in joint fluid confirms that the infection has affected the joint, which is crucial for diagnosing Lyme disease arthritis.

How long does it take to get results?

Immunoblot testing is more complex than standard blood tests and typically takes 3-7 days to complete. The laboratory must process the synovial fluid sample through multiple steps to detect the specific antibodies.

Do I need to fast before this test?

No fasting is required for this test. However, you should inform your doctor about any medications you are taking, as some may affect joint fluid composition.

What happens after a positive result?

If positive, your healthcare provider will typically prescribe a course of oral or intravenous antibiotics depending on the severity of symptoms. Most patients respond well to antibiotic treatment for Lyme disease arthritis.

Can this test be done if I have no joint swelling?

This test is specifically designed for synovial fluid analysis and is only performed when there is sufficient joint fluid to aspirate, typically when a patient presents with observable joint swelling or effusion.

Sources

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This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy