Purpose: Mycoplasma pneumoniae (MP) is a common cause of community-acquired pneumonia (CAP) in children. MP serum IgM and polymerase chain reaction (PCR) are the methods that enable early diagnosis in patients with MP pneumonia. The objective of this study was to investigate the clinical value of serum MP-specific IgM antibodies in PCR-positive MP pneumonia for the early diagnosis of MP pneumonia in children with CAP. Methods: Out of 129 patients with lower respiratory tract infection aged over

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... 3 years, 90 CAP children were enrolled in the study. Throat swab MP real-time PCR and serum enzyme-linked immunosorbent assays (ELISA) IgM antibodies were performed. A positive rate of MP PCR and serum IgM, the level of IgM index, clinical features, and laboratory findings were analyzed. Results: MP PCR was positive in 57 cases. Longer fever duration before admission (P < 0.001), higher rates of lobar or segmental pneumonia (P= 0.048), unilateral infiltration (P= 0.038), and extrapulmonary symptoms (P= 0.049) were associated with MP PCRpositive pneumonia. Serum IgM index was significantly higher in MP PCR-positive pneumonia them in MP PCR-negative pneumonia (3.9± 3.0 vs. 0.8± 1.3, P< 0.001). Using MP PCR as a gold standard, the sensitivity, specificity, positive predictive value and negative predictive value of serum IgM were 85.5%, 82.1%, 91.4%, and 71.9%, respectively. The area under the curves for serum IgM index was 0.892, and the ROC analysis indicated that an optimal cutoff value of 1.05 for serum IgM provided the highest sensitivity and specificity interestingly (83.9% vs. 85.7%, P< 0.001). Conclusion: Serum IgM ELISA has useful diagnostic value in PCR-positive MP pneumonia. Applying an IgM index cutoff of 1.05 improves diagnostic accuracy. (Allergy Asthma Respir Dis 2018;6:248-254)