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 a parts per thousand yen65 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.