IgG Outer Surface P. A+C

What is it?

The IgG Outer Surface Proteins A and C (OspA & OspC) test is a blood test that measures your immune system’s production of IgG antibodies against specific proteins on the surface of Borrelia burgdorferi, the bacteria that causes Lyme disease.

Outer Surface Proteins (Osp) are molecules the bacteria use to survive and infect humans:

• OspA helps the bacteria persist in ticks.

• OspC plays a key role in establishing infection in people.

When your immune system encounters Borrelia burgdorferi, it may produce IgG antibodies targeting these proteins. Detecting these antibodies can help provide clues about whether your body has mounted a response to Lyme disease.

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Why is this marker important?

• A positive IgG OspA/OspC result suggests that your body has been exposed to the Lyme bacteria in the past and created antibodies.

• IgG antibodies usually appear weeks to months after infection, so they are more commonly seen in later or persistent stages of Lyme disease.

• This test is often used alongside other Lyme disease tests (such as IgM antibodies, ELISA, or Western blot) to give a more complete diagnostic picture.

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How to interpret results

Elevated IgG OspA/OspC

• Suggests past exposure or possible ongoing infection with Borrelia burgdorferi

• Typically appears in later or chronic phases of the disease

• Levels may stay elevated for years, even after successful treatment

Normal or Negative IgG OspA/OspC

• May mean no prior exposure

• Could also mean antibodies have not yet developed (common in very early Lyme disease, known as the “window period”)

• Repeat testing may be considered if symptoms suggest Lyme but the initial test is negative

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Symptoms often linked with Lyme disease

People who test positive for Lyme markers often recall symptoms such as:

• Early stage: bull’s-eye rash (erythema migrans), fever, chills, muscle aches, fatigue

• Later stage: joint pain, headaches, nerve pain, memory or concentration problems, irregular heartbeat

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What can affect this test?

• Stage of infection: IgG develops later, so it’s not reliable for early detection

• Immune system health: Those with weakened immunity may not produce strong antibodies

• Previous infection: IgG antibodies may persist long after treatment and recovery

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What to do if your results are elevated

An elevated result by itself does not confirm active Lyme disease. Your healthcare provider will interpret these findings together with:

• Your symptoms and medical history

• Other Lyme tests (IgM, ELISA, Western blot, or PCR)

• Possible tick exposure in Lyme-endemic areas

If Lyme disease is suspected, your doctor may recommend antibiotic treatment tailored to the stage and presentation of the illness.

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Key takeaway

The IgG OspA/OspC test shows whether your immune system has produced longer-term antibodies to Lyme disease bacteria. A positive result indicates past or possible ongoing exposure, but it does not by itself prove an active infection. Your doctor will combine this result with your symptoms, exposure history, and additional tests to make the most accurate diagnosis and treatment plan.