Investigating the biomarkers associated with pathogenicity and colonization of lower respiratory tract pathogens through analysis of bronchoalveolar lavage fluid

Objective This study aims to ascertain the optimal volume and procedural protocol for bronchoscopic bronchoalveolar lavage (BAL) in patients with lower respiratory tract infection (LRTI), improve diagnostic yield, and identify cell-component-based biomarkers in bronchoalveolar lavage fluid (BALF) that can distinguish active infection from microbial colonization. Methods This study is comprised of two distinct phases. In Part 1, a cohort of 130 patients was enrolled, including 96 individuals with confirmed LRTI and 34 without LRTI. Under bronchoscopic guidance, two sequential 60 mL aliquots of normal saline (total volume, 120 mL) were instilled into the target segment. BALF was collected separately after each 60 mL instillation. The microbial yield and cellular composition of each aliquot were analyzed to ascertain the optimal volume and procedural protocol for BAL in patients with LRTI. In Part 2, of these 130 patients, 110 with suspected LRTI were further classified based on clinical and microbiological criteria: 96 were identified as having a pathogenic infection, while 14 were determined to have microbial colonization. BAL was performed using the optimal volume and method established in Part 1. The BALF obtained was analyzed to identify cellular biomarkers capable of distinguishing active infection from microbial colonization. The diagnostic efficacy of these biomarkers was assessed. Results In Part 1, patients with LRTI exhibited elevated peripheral blood neutrophil counts and higher CRP and IL-6 levels. Large-volume BALF (60-120mL) revealed a greater number of significantly different parameters (9 in Large-volume vs 8 in Small-volume;P<0.05). In Part 2, the pathogenic group had higher serum IL-6 levels than the colonized group. Analysis of BALF showed higher neutrophils and polymorphonuclear cells, whereas macrophages, CD206 + HLA-DR + macrophages (tissue-resident macrophages), mononuclear cells, and tissue-resident macrophages to CD14 low CD16 + monocytes ratio (TRMMR) were lower (P<0.05). The AUCs for predicting infection were 0.668 for CD16 + CD24 + neutrophils, 0.673 for polymorphonuclear cells, and 0.693 for the combined model. The AUCs for predicting colonization were 0.682 for macrophages, 0.687 for CD206 + HLA-DR + macrophages, 0.676 for TRMMR, and 0.681 for mononuclear cells. Conclusion Large-volume BALF is optimal for cellular analysis in LRTI. Elevated proportions of neutrophils and polymorphonuclear cells may indicate infection, whereas increased proportions of macrophages (particularly CD206 + HLA-DR + subset), mononuclear cells, and TRMMR were associated with colonization. These cellular indices may serve as valuable biomarkers for differentiating infection from colonization.