Dynamics of Borrelial Antibodies in European Lyme Borreliosis Patients with Erythema Migrans.

Background/Objectives: The diagnosis of typical erythema migrans (EM) is clinical, and serological testing for Borrelia-specific antibodies is not required. However, characterization of the immune response in patients with a reliable clinical diagnosis may improve the interpretation of serological findings in early Lyme borreliosis (LB) without characteristic skin lesions. Methods: Serum IgM and IgG antibodies to Borrelia burgdorferi sensu lato were evaluated in 1356 Slovenian adults with EM at baseline and at a follow-up visit approximately two months after treatment initiation, using a chemiluminescence immunoassay based on recombinant borrelial antigens. Results: At baseline, 41.5% of patients were IgM-seropositive, 55.5% IgG-seropositive, and 70.1% positive for either IgM or IgG. Patients with multiple EM were significantly more often seropositive than those with solitary EM. In previously untreated patients, baseline IgG seropositivity showed a clearer positive association with longer lesion duration and larger lesion diameter, whereas the corresponding IgM patterns were less consistent. Multiple EM remained strongly associated with both IgM and IgG seropositivity in adjusted analyses. Seroconversion at follow-up was uncommon, occurring in 8.0% of initially IgM-negative and 8.2% of initially IgG-negative patients. Conclusions: The low frequency of newly detectable antibody responses indicates that repeat qualitative serologic testing approximately two months after treatment initiation provides limited additional diagnostic information in European adults with EM.