Evaluation of methylene blue and squamous epithelial cells as oropharyngeal markers: a means of identifying oropharyngeal contamination during transtracheal aspiration.

The usefulness of 1% methylene blue (MB) and squamous epithelial cells as oropharyngeal markers in transtracheal aspiration was prospectively evaluated. In vitro studies showed that failure to detect MB by spectrophotometry ruled out contamination by greater than 0.05 microliters of oropharyngeal secretions and that visual inspection was almost as sensitive as spectrophotometry. Even minute contamination could be ruled out if greater than 5 x 10(4) organisms were found by culture of Gram-stained smear in a specimen that was MB-negative by spectrophotometry. In specimens of transtracheal aspirate obtained from 10 bronchitic patients, quantitative bacteriology ruled out even minute contamination in nine. Cytologic-morphometric examination revealed that 70% of both sterile and colonized specimens of transtracheal aspirate contained squamous epithelial cells that were indistinguishable, except by electron microscopy, from buccal mucosal cells. MB is a useful marker for identification of oropharyngeal contamination during transtracheal aspiration, and traditional cytologic screening is misleading in conditions associated with tracheobronchial squamous metaplasia.