Characteristics and influencing factors of bronchoalveolar lavage fluid cellular patterns in children with Mycoplasma pneumoniae pneumonia: a cross-sectional study

Background: Mycoplasma pneumoniae pneumonia (MPP) is a common cause of community-acquired pneumonia in children. Clinical assessment currently relies mainly on blood inflammatory markers. Bronchoalveolar lavage fluid (BALF) cellular analysis provides an important window to directly assess local pulmonary inflammation. Small studies have demonstrated BALF neutrophil predominance in pediatric MPP. However, large-scale studies on the characteristics and influencing factors of BALF cellular patterns are lacking. In particular, it remains unclear whether local MP-DNA load independently associates with neutrophil predominance or whether common blood inflammatory markers correlate with BALF cellular patterns. This large cross-sectional study aimed to characterize BALF cellular patterns and investigate influencing factors.

Methods: This single-center cross-sectional study consecutively enrolled children with confirmed MPP (positive BALF MP-DNA) hospitalized at Tianjin Children’s Hospital from November 2023 to April 2024. Diagnosis followed the Chinese Guidelines for the Diagnosis and Treatment of Mycoplasma pneumoniae Pneumonia in Children (2023 Edition). BALF cytological examination were performed in accordance with the Consensus of Chinese Experts on Morphological Examination of Bronchoalveolar Lavage Fluid Cells [2023]. Clinical and laboratory covariates were extracted from medical records. Spearman’s rank correlation, Kruskal-Wallis H test, Mann-Whitney U test and multivariable linear regression were used for statistical analysis.

Results: (I) Among 238 children (mean age 7.79±2.76 years; 49.2% male), median BALF neutrophil proportion was 80.0% [interquartile range (IQR), 71.8%, 84.0%], lymphocyte proportion 3.0% (IQR, 2.0%, 5.0%), and macrophage proportion 13.5% (IQR, 11.0%, 19.0%). (II) Univariable analysis showed a positive correlation between BALF MP-DNA load and neutrophil proportion (rs =0.268, P<0.001). Multivariable linear regression confirmed that MP-DNA load remained independently associated with BALF neutrophil proportion [B=5.435; 95% confidence interval (CI): 3.073–7.796; P<0.001]. The high MP-DNA load group (≥10⁷ copies/mL) had a significantly higher neutrophil proportion than the medium‑load group (Z=−3.924, P<0.001). (III) BALF neutrophil proportion was not significantly correlated with common blood inflammatory markers [leukocyte, neutrophil, lymphocyte counts, procalcitonin (PCT), lactate dehydrogenase (LDH), and ferritin] in univariable analysis (all P>0.05). C-reactive protein (CRP) showed a weak univariable correlation (rs =0.144, P=0.03), but it was not independently associated in multivariable analysis (P=0.17). (IV) Exploratory analysis of pleural effusion indicated its presence was associated with a lower BALF lymphocyte proportion in univariable analysis (Z=−2.093, P=0.04); multivariable regression was not feasible due to limited cases (n=38).

Conclusions: In children with MPP, BALF cellular patterns are characterized by neutrophil predominance, which independently associated with local MP-DNA load, but not with most common blood inflammatory markers. The findings indicate BALF cellular patterns provide complementary information for assessing pulmonary immune status and inflammatory phenotypes in pediatric MPP, while not establishing causality or therapeutic benefit.