Admission-time immunologic patterns in hospitalized children with Mycoplasma pneumoniae pneumonia: a molecular...

Targeted next-generation sequencing (tNGS) provides a semi-quantitative signal of Mycoplasma pneumoniae (MP) molecular burden, whereas antibody titers reflect humoral immune response. However, the joint relationship between MP molecular load, antibody response, and illness timing remains poorly characterized in hospitalized children with MP pneumonia (MPP). We described tNGS-based co-detection profiles in the full cohort and performed admission-time load-titer phenotyping in children with MP-only pneumonia.

We conducted a retrospective cohort study of hospitalized children with tNGS-confirmed MPP from January to December 2024. Co-detection profiles and associated clinical characteristics were summarized in the full cohort. Formal load-titer phenotyping was restricted to children with MP-only pneumonia to reduce co-detection-related heterogeneity. In this subgroup, two-dimensional kernel density estimation (2D-KDE) was applied to log₁₀-transformed normalized MP reads and log₂(titer + 1)-transformed antibody titers. Onset-to-admission intervals were also summarized across antibody titer categories.

Among 402 children, MP-only, MP + viral co-detection, and MP + bacterial co-detection accounted for 39.3%, 30.1%, and 30.6% of children, respectively. MP + viral co-detection was associated with younger age, longer cough duration, higher white blood cell (WBC) and platelet counts, lower C-reactive protein (CRP), higher antibody titers, and slightly greater resource use, whereas MP + bacterial co-detection generally resembled MP-only. Antibody titers showed a timing-related gradient: children with the first positive titer level of 1:40 were typically admitted around day 6 after illness onset, whereas very high titers corresponded to longer onset-to-admission intervals. Among 158 children with MP-only pneumonia, 2D-KDE suggested three admission-time molecular-serologic patterns: P1 high-load/seronegative, P2 high-load/high-titer, and P3 lower-load/high-titer, accounting for 29.7%, 45.6%, and 24.7% of children, respectively. Across P1-P3, onset-to-admission interval and fever duration increased, the neutrophil-to-lymphocyte (N/L) ratio rose, and platelet counts peaked in P2 before partially declining in P3.

Admission-time molecular load-antibody titer phenotyping in children with MP-only hospitalized MPP suggested three immunologic patterns. The high-load/high-titer pattern indicates that persistent MP molecular signal and established humoral response may coexist at hospitalization. This joint framework may help clinicians contextualize MP tNGS signals in relation to host immune status and illness timing, rather than relying on molecular load or antibody titer alone.