Serodiagnosis of Lyme borreliosis—is IgM in serum more harmful than helpful?

Interpretation of serological findings in suspected Lyme borreliosis (LB) may be challenging and IgM reactivities in serum are often unspecific (false positive). There is a risk for overdiagnosis of LB, inadequate use of antibiotics, and potential delay of proper diagnosis. In this study, we evaluated the diagnostic value of IgM analysis in serum and IgM antibody index (AI) in LB diagnosis. This was a retrospective observational study regarding Borrelia-specific antibodies in serum and Borrelia-specific AI in LB investigations being made during 2017 in Jönköping County, Sweden. Medical records of 610 patients with detectable anti-Borrelia antibodies in serum (IgM and/or IgG) and 15 patients with elevated Borrelia-specific AI were retrospectively scrutinized, and the compliance to current European recommendations was assessed. Among the 610 patients, only 30% were tested according to the European recommendations. Within this group of tests taken correctly, 50% of the LB diagnoses in patients with isolated IgM reactivity in serum were retrospectively assessed as incorrect (LB unlikely). Three pediatric patients with clinical and laboratory findings suggestive of Lyme neuroborreliosis (LNB) had elevated IgM AI alone. Serological testing without distinct clinical signs/symptoms consistent with LB contributes to most misdiagnoses. Isolated IgM positivity in serum shows limited clinical value and needs further assessment before being reported by the laboratory. Detection of IgM in combination with IgG antibodies in serum shows no clinical enhancement for correct LB diagnosis compared to isolated IgG positivity. However, Borrelia-specific IgM AI may be important for sensitivity in early LNB.