Bacteriological Correlation of Sputum and Bronchoalveolar Lavage Cultures in Suspected Cases of Lower Respiratory Tract Infections

Introduction: Lower respiratory tract infections (LRTIs) are a leading cause of global morbidity and mortality, necessitating accurate microbiological diagnosis for effective antimicrobial stewardship. Sputum is the primary non-invasive diagnostic specimen, but its reliability is often limited by oropharyngeal contamination, leading to frequent false-negative results. Bronchoalveolar lavage (BAL) provides a more representative sample of the lower airway but is invasive. This study aimed to evaluate the bacteriological correlation between sputum and BAL cultures and describe associated antimicrobial susceptibility patterns in patients with suspected LRTIs.

Materials and Methods: A descriptive cross-sectional study was conducted on 140 adult patients (≥18 years) with suspected LRTIs in the Department of Respiratory Medicine at the Pacific Institute of Medical Sciences (Udaipur, India) between July 2023 and December 2024. Paired expectorated sputum and BAL samples were collected simultaneously and processed for aerobic culture and antimicrobial susceptibility testing (Kirby–Bauer disk diffusion method) following CLSI-2019 guidelines. The quality of sputum samples was microscopically assessed before culture.

Results: The mean age of participants was 54.98 pm 13.53 years, with a pronounced male predominance (90.71%). Chronic lung disease (47.86%) and smoking (64.29%) were the most common comorbidity and risk factor, respectively. BAL demonstrated significantly higher culture positivity (80.71%) compared to sputum (25.00%). Pseudomonas aeruginosa was the most common isolate in both samples, but its detection rate was significantly higher in BAL (27.86%) than in sputum (7.14%) (p<0.001). Similar highly significant differences were noted for Acinetobacter baumannii (BAL: 15.00%, Sputum: 2.14%; p<0.001) and Escherichia coli (BAL: 12.14%, Sputum: 1.43%; p<0.001). Isolates from both samples showed substantial antimicrobial resistance, particularly Gram-negative organisms. In BAL, Acinetobacter baumannii showed resistance to macrolides in all detected cases, while E. coli was most sensitive to beta-lactams.

Conclusion: Sputum culture exhibits a high false-negative rate (75%) compared to BAL in diagnosing LRTIs, severely underestimating the prevalence of Gram-negative pathogens like P. aeruginosa and A. baumannii. BAL is superior for definitive microbiological diagnosis, particularly in a population with high prevalence of chronic lung disease and exposure risk factors. These findings reinforce the need for targeted antibiotic policies guided by lower airway sampling, especially in cases of severe illness or where sputum results are inconclusive.