Prolonged Culture Negativity in Infective Endocarditis Following Short-Term Antibiotic Exposure: A Case Report

Blood culture-negative endocarditis remains diagnostically challenging, especially after antibiotic exposure. We report the case of a 63-year-old woman with culture-negative infective endocarditis (IE), in whom cultures were likely rendered negative due to prior antibiotic therapy consisting of levofloxacin and ceftriaxone for 12 days more than 30 days earlier and two doses of cefazolin administered 12 days before presentation. All blood and valve cultures were negative, but 16S rRNA polymerase chain reaction (PCR) identified Streptococcus mitis (group), guiding targeted therapy. This case illustrates that antibiotic exposure more than 10 days prior to sampling may still yield negative cultures and emphasizes the diagnostic utility of molecular tools. Clinicians should consider blood culture-negative endocarditis even after extended antibiotic-free periods.