Background: The administration of antibiotics prior to specimen collection may reduce the yield of culture-based bacterial detections. The effect on culture-independent diagnostic tests remains uncertain. This study assesses the impact of antibiotic exposure on both culture-based and culture-independent test results among patients hospitalized with CAP. <br /><br />Methods: The study evaluated 4,678 patients hospitalized with CAP. Culture-based tests included blood cultures and high-quality sputum or endotracheal aspirates. Culture-independent tests encompassed urinary antigen testing for Streptococcus pneumoniae and Legionella pneumophila, as well as PCR assays on nasopharyngeal and oropharyngeal swabs for Mycoplasma pneumoniae and Chlamydia pneumoniae. The proportion of bacterial detections was compared between specimens collected before and after antibiotic exposure, considering both prehospital and inpatient administration. <br /><br />Results: Among the patients, 94% received antibiotics: 85% only inpatient, 15% both inpatient and prehospital, and less than 1% only prehospital. Bacterial detections were higher in specimens collected before antibiotic administration for blood cultures (5.2% vs. 2.6%; P < .01) and sputum/ET cultures (50.0% vs. 26.8%; P < .01). No significant differences were observed in urinary antigen tests (7.0% vs. 5.7%; P = .53) or PCR assays (6.7% vs. 5.4%; P = .31). Additionally, bacterial detections declined with increasing time between antibiotic administration and specimen collection. <br /><br />Conclusions: Antibiotic exposure is associated with reduced bacterial detection in culture-based tests and, to a lesser extent, in culture-independent tests. Prompt specimen collection, preferably before antibiotic administration, may enhance diagnostic yield and inform targeted antibiotic therapy.<br /><br />⸻<br /><br />This study underscores the importance of timing in specimen collection relative to antibiotic administration to maximize diagnostic accuracy in CAP.<br />د. سجاد علوان كتاب <br />Al-Mustaqbal University is the first university in Iraq<br /><br />