Sputum plug selection under inverted microscopy improves microbial identification during exacerbations of airway diseases

BACKGROUND: Salivary contamination decreases the yield of identifying potential airway bacterial and fungal pathogens in routine cultures of spontaneous or induced sputum during exacerbations of airway diseases. We investigated the utility of selecting out the squamous cells from sputum using inverted microscopy. METHODS: Sputum was obtained from fifty subjects, then divided to facilitate parallel processing of paired samples for gram-stain and semi-quantitative cultures, and quantitative cytometry. RESULTS: Selective processing under inverted microscopy allowed for fewer sample rejections (2% vs. 24%) and greater culture positivity (58% vs. 24%) compared with routine practices, with significant yield of fungal organisms. While known pathogens were associated with intense sputum neutrophilia, organisms that were not routinely considered as airway pathogens were also associated with modest neutrophilia indicating that they are capable of inducing a clinically relevant cellular response. CONCLUSION: Sputum selection under inverted microscopy improves the detection of bacterial pathogens during infective exacerbations. The methods to assign pathogenicity to microbes in the airway needs to be re-visited by assessing the cellular responses that they evoke in the airways.