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

## TL;DR

A patient with a history of antibiotic use had blood culture-negative endocarditis, but molecular testing identified the bacteria, highlighting the usefulness of such tools in diagnosis.

## Contribution

Demonstrates that prior antibiotic exposure over 10 days can still lead to culture-negative endocarditis and underscores the value of molecular diagnostics.

## Key findings

- Antibiotic exposure more than 10 days prior can result in culture-negative endocarditis.
- 16S rRNA PCR identified Streptococcus mitis despite negative cultures.
- Molecular tools are essential for diagnosing culture-negative cases.

## Abstract

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.