A 42-year-old man with intravenous drug use presents with fever, malaise, and a new diastolic murmur. Clinical suspicion for infective endocarditis is high. Blood cultures are pending. Which investigation should be performed next to establish the microbiological diagnosis and guide antibiotic therapy?

## Microbiological Diagnosis of Infective Endocarditis **Key Point:** Blood cultures are the gold standard for microbiological diagnosis of infective endocarditis and must be obtained before initiating antibiotic therapy to identify the causative organism and determine antimicrobial susceptibility. ### Blood Culture Protocol in Suspected IE **High-Yield:** The standard approach is: 1. Obtain **3 sets of blood cultures** (each set = aerobic + anaerobic bottles) 2. Draw from **different venipuncture sites** to distinguish contamination from true bacteremia 3. Obtain cultures **before antibiotics** whenever possible — delaying antibiotics by 1–2 hours for proper culture collection does not worsen outcomes 4. If patient is already on antibiotics, wait 48 hours before repeat cultures (allows washout) ### Why Blood Cultures Are Essential | Aspect | Importance | |--------|------------| | Organism identification | Determines specific antibiotic choice | | Susceptibility testing | Guides dosing and duration of therapy | | Prognosis | Certain organisms (e.g., *Staph. aureus*) have worse outcomes | | Complications | Some organisms predispose to abscess or emboli | | Treatment duration | Typically 4–6 weeks; some organisms require longer | **Clinical Pearl:** In IVDU with IE, *Staphylococcus aureus* (including MRSA) is the most common causative organism, followed by streptococci. Blood cultures will identify the specific pathogen and methicillin resistance status, which is critical for choosing between nafcillin and vancomycin. ### Modified Duke Criteria Positive blood cultures are a **major criterion** in the diagnosis of IE: - Typical microorganism isolated from blood cultures - Microorganism consistent with IE from persistently positive cultures **Mnemonic: HACEK** — *Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella* — fastidious Gram-negative organisms that cause IE and require prolonged culture incubation (up to 3 weeks). **Warning:** Do not delay blood culture collection to perform imaging or other investigations. Cultures must be obtained first, before antibiotics are started, to maximize diagnostic yield. [cite:Harrison 21e Ch 124]