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.

对疑似莱姆疏螺旋体病 (LB) 的血清学发现进行解释可能具有挑战性，并且血清中的 IgM 反应性通常是非特异性的（假阳性）。存在过度诊断 LB、抗生素使用不当以及可能延误正确诊断的风险。在本研究中，我们评估了血清 IgM 分析和 IgM 抗体指数 (AI) 在 LB 诊断中的诊断价值。这是2017 年在瑞典延雪平县进行的关于血清中疏螺旋体特异性抗体和LB 调查中疏螺旋体特异性 AI的回顾性观察研究。610 名血清中可检测到抗疏螺旋体抗体（IgM 和/或 IgG）的患者和 15 名升高的疏螺旋体患者的医疗记录对特定的 AI 进行了回顾性审查，并评估了对当前欧洲建议的遵守情况。在 610 名患者中，只有 30% 根据欧洲建议进行了检测。在这组正确进行的测试中，在血清中具有孤立 IgM 反应性的患者中，50% 的 LB 诊断被回顾性评估为不正确（LB 不太可能）。三名临床和实验室检查结果提示莱姆神经疏螺旋体病 (LNB) 的儿科患者仅 IgM AI 升高。没有与 LB 一致的明显临床体征/症状的血清学检测导致大多数误诊。血清中孤立的 IgM 阳性显示出有限的临床价值，在实验室报告之前需要进一步评估。与孤立的 IgG 阳性相比，检测 IgM 与血清中的 IgG 抗体并没有显示对正确 LB 诊断的临床增强。然而，疏螺旋体特异性 IgM AI 可能对早期 LNB 的敏感性很重要。