Effect of antimicrobial use on pneumococcal diagnostic tests in elderly patients with community-acquired pneumonia

Antimicrobial treatment decreases bacterial culture yields. We assessed the impact of antimicrobial treatment on pneumococcal assays in a prospective study of community-acquired pneumonia (CAP) in the elderly. We enrolled 323 cases aged ≥65 years with radiologically confirmed CAP and collected detailed data on antimicrobial exposure and pneumococcal assays on various samples. Complete antimicrobial use data were available for 303 (94 %) cases; 61 % had no antimicrobial exposure, 19 % had received antibiotics at the acute visit only, and 20 % within 2 weeks before the acute visit (15 % ongoing and 5 % completed treatment). Ongoing use before the visit reduced pneumococcal detection by culture (nasopharyngeal swab 2 vs. 16 % in the unexposed; high-quality sputum 0 vs. 25 %) and sputum lytA polymerase chain reaction (PCR) (0 vs. 25 %). Urine antigen test and serology were not affected. Among those who had received antibiotics only at the acute visit before study sampling, serology (29 vs. 15 %), urine antigen (19 vs. 8 %), and blood culture (9 vs. 2 %) were more often positive than among the unexposed. Antimicrobial exposure before the visit reduced both culture and PCR-based detection. Patients given antibiotics at the visit had higher proportions of positive blood culture, serology, and urine antigen tests, suggesting higher pneumococcal CAP prevalence.