IgM Outer Surface Protein A + C (OspA + OspC)
The IgM OspA + OspC test detects early immune responses to Borrelia burgdorferi, the bacteria responsible for Lyme disease. It looks for IgM antibodies against two key surface proteins:
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OspC (Outer Surface Protein C): Produced in the early stages of infection.
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OspA (Outer Surface Protein A): More often associated with later or persistent infection.
Because Borrelia expresses these proteins at different points in its life cycle, testing for both can provide a broader picture of the stage and progression of Lyme disease.
What the Results Mean
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OspC IgM antibodies: Typically appear within 2–3 weeks after a tick bite, making them a strong marker for recent Lyme infection.
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OspA IgM antibodies: More often linked with later or persistent disease, and sometimes with chronic or autoimmune-like manifestations of Lyme.
When considered together, OspA and OspC testing helps clinicians estimate where a patient may be along the Lyme disease timeline.
Clinical Importance
This test may be most helpful for patients who present with:
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Early Lyme symptoms such as fever, rash (erythema migrans), fatigue, headache, or muscle aches
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Neurological involvement including facial palsy, meningitis-like symptoms, or nerve pain
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Joint pain or swelling that develops weeks to months after exposure
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Documented tick bites or exposure in areas where Lyme disease is common
Limitations
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IgM reliability is time-sensitive: IgM is most useful in the early weeks of infection. Isolated IgM positivity months later may not indicate active infection.
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False positives can occur due to cross-reactivity with other bacterial or viral infections.
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Confirmatory testing is standard: Additional markers such as IgG antibodies, VlsE/C6 peptide, or multi-antigen panels are often used to strengthen diagnostic accuracy.
Patient Takeaway
A positive IgM OspA + OspC test means your immune system is producing early antibodies against Borrelia, suggesting possible recent or ongoing exposure to the bacteria that causes Lyme disease. However, this test alone does not confirm Lyme disease.
Your doctor will always interpret the results in the context of:
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Your symptoms
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Tick exposure history
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Other Lyme disease test results
If your test is reported as equivocal (borderline), it means antibody levels are close to the cutoff and not clearly positive or negative. In these cases, your provider may recommend repeat testing or additional confirmatory panels.
IgM Outer Surface P. A+C
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