Throat Swabs and Sputum Culture as Predictors of P. aeruginosa or S. aureus Lung Colonization in Adult Cystic Fibrosis Patients

BACKGROUND Due to frequent infections in cystic fibrosis (CF) patients, repeated respiratory cultures are obtained to inform treatment. When patients are unable to expectorate sputum, clinicians obtain throat swabs as a surrogate for lower respiratory cultures. There is no clear data in adult subjects demonstrating the adequacy of throat swabs as a surrogate for sputum or BAL. Our study was designed to determine the utility of throat swabs in identifying lung colonization with common organisms in adults with CF. METHODS Adult CF subjects (n = 20) underwent bronchoscopy with BAL. Prior to bronchoscopy, a throat swab was obtained. A sputum sample was obtained from subjects who were able to spontaneously expectorate. All samples were sent for standard microbiology culture. RESULTS Using BAL as the gold standard, we found the positive predictive value for Pseudomonas aeruginosa to be 100% in both sputum and throat swab compared to BAL. However, the negative predictive value for P. aeruginosa was 60% and 50% in sputum and throat swab, respectively. Conversely, the positive predictive value for Staphylococcus aureus was 57% in sputum and only 41% in throat swab and the negative predictive value of S. aureus was 100% in sputum and throat swab compared to BAL. CONCLUSIONS Our data show that positive sputum and throat culture findings of P. aeruginosa reflect results found on BAL fluid analysis, suggesting these are reasonable surrogates to determine lung colonization with P. aeruginosa. However, sputum and throat culture findings of S. aureus do not appear to reflect S. aureus colonization of the lung.