Background: The incidence of severe Mycoplasma pneumoniae pneumonia (SMPP) in children has markedly increased in the post-COVID-19 era. Current diagnosis approaches often rely on persistent fever, which may delay early identification. This study aims to evaluate the synergistic value of Lactate Dehydrogenase (LDH) and Interleukin-6 (IL-6) for early stratification.

Methods: We retrospectively analyzed clinical data from 210 hospitalized children with Mycoplasma pneumoniae pneumonia (MPP; 120 severe and 90 non-severe). Independent risk factors were identified using multivariate logistic regression to build a dynamic nomogram. To validate the model's early warning capability and exclude circularity, a specific subgroup analysis was performed on patients admitted in the early phase of illness (fever duration <7 days).

Results: LDH, IL-6, and pre-admission fever duration were independent predictors. The combined nomogram outperformed single biomarkers with an Area Under the Curve (AUC) of 0.827. Crucially, in the early-phase subgroup, both IL-6 and LDH remained significantly elevated in SMPP cases (p < 0.05), proving that pathological signals precede the clinical threshold for severity. The study revealed a “two-axis” mechanism whereby IL-6 quantified systemic inflammation, while LDH reflected local tissue hypoxia (e.g., atelectasis), distinct from the systemic response. Decision Curve Analysis further confirmed the model's clinical utility.

Conclusion: This study proposes a dual-pathology stratification model where IL-6 reflects cytokine storm and LDH indicates tissue injury. The nomogram reliably identifies high-risk children in the early illness phase, facilitating intervention before persistent fever develops.