Testing for IgM antibodies to Borrelia burgdorferi in Scottish patients with suspected Lyme borreliosis was introduced in 2018 to supplement the IgG testing already in situ. Results from 2018 to 2020 were assessed alongside available clinical data to evaluate the utility of IgM testing in serum. An estimated false positive rate of 25.5% was observed with IgM immunoblot vs 80.1% for IgM chemiluminescent immunoassay (CLIA). IgM testing can aid earlier diagnoses if used within a selective two-tier testing protocol: only patients with acute onset of symptoms should be tested for IgM CLIA but confirmation by immunoblot and consideration of clinical picture is necessary.

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