The impact of sputum sample quality on sputum culture results: a retrospective analysis of clinical microbiology sputum culture data spanning 10 years.

BACKGROUND: Despite advances in molecular diagnostics, sputum culture remains fundamental for the microbiological diagnosis of lower respiratory tract infections. Current guidelines recommend rejecting unacceptable sputum specimens, yet their actual diagnostic yield, pathogen distribution, and impact on antimicrobial resistance assessment have not been systematically evaluated using large-scale data. A retrospective analysis was performed on 143,101 sputum culture and quality assessment records collected at Nanjing Drum Tower Hospital between 2015 and 2025. Specimens were classified as acceptable or unacceptable according to the Murray-Washington criteria, with the same period bronchoalveolar lavage fluid cultures included as comparator group.

RESULTS: The overall culture-positivity rate was significantly higher in acceptable than in unacceptable specimens (57.28% vs. 50.87%; Odds Ratio = 1.29, 95% CI: 1.27-1.32, P < 0.001). Dominant organisms, including Klebsiella pneumoniae and Acinetobacter baumannii, were detected at higher rates in acceptable specimens, whereas yeast-like fungi were significantly enriched in unacceptable sputum, indicating oropharyngeal contamination. Filamentous fungi (e.g., Aspergillus spp.) showed a stepwise decreasing trend in BALF > acceptable sputum > unacceptable sputum. Antimicrobial resistance rates of K. pneumoniae, Pseudomonas aeruginosa, and Staphylococcus aureus were significantly higher in sputum isolates than in BALF isolates, suggesting that sputum cultures may overestimate resistance risk.

CONCLUSION: The large-scale findings support cautious, contextual interpretation of unacceptable sputum results, particularly when better specimens cannot be obtained. This approach helps preserve critical diagnostic information and avoid unnecessary antimicrobial escalation.