Quality evaluation of sputum specimens for mycobacterial culture

The evaluation of sputum specimen quality before bacterial culture is an accepted practice. Traditionally, sputum received for mycobacterial culture has been processed regardless of specimen quality. In view of the increasing emphasis placed on the primary acid-fast smear, the effect of specimen quality and the contribution of the presence of neutrophils on subsequent smear and culture positivity for mycobacteria was assessed. A total of 873 sputa were evaluated and initially assigned a quality (Q) score of 1 (many squamous cells relative to neutrophils) to 3 (no squamous cells) based on criteria for specimen acceptability for routine bacterial culture. The percentage of specimens that were Q1, Q2, and Q3 were 46.8, 35.3, and 17.9, respectively. Most of the specimens received were Q1, and these specimens demonstrated the highest number of positive smears and cultures compared to Q2 or Q3 specimens. Thus, routine bacterial culture criteria were not helpful in assessing specimen quality for mycobacterial culture. The contribution of the presence of neutrophils to smear and culture positivity was assessed. A total of 724 sputa were evaluated for cellular composition: 665 (91.9%) specimens contained neutrophils, and 59 (8.1 %) did not contain neutrophils. A total of 51 (7.0%) primary smears and 121 (16.7%) cultures were positive for mycobacteria; 92.2% of the positive smears; and 90.1 % of the positive cultures were from specimens that contained neutrophils. Thus, screening sputum specimens for the presence of neutrophils provides an effective method to evaluate the acceptability of sputum for mycobacterial smear and culture, especially from patients in respiratory isolation.