What are the laboratory studies for diagnosing infective endocarditis?

From the Guidelines

Laboratory studies for endocarditis should include blood cultures, complete blood count, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and basic metabolic panel, with three sets of blood cultures drawn from different sites at least 30 minutes apart before starting antibiotics, as recommended by the most recent guidelines 1.

Key Laboratory Tests

• Blood cultures: cornerstone of diagnosis, with three sets drawn from different sites at least 30 minutes apart before starting antibiotics
• Complete blood count: typically shows leukocytosis and anemia
• ESR and CRP: indicate ongoing inflammation
• Rheumatoid factor: may be positive in subacute endocarditis
• Cardiac biomarkers like troponin: assess for myocardial involvement
• Urinalysis: may reveal microscopic hematuria or proteinuria due to immune complex deposition

Specialized Testing

If blood cultures remain negative after 48-72 hours but endocarditis is still suspected, specialized testing for fastidious organisms like HACEK group bacteria, Bartonella, Coxiella, or fungi may be necessary, as suggested by the American Heart Association 1.

Empirical Therapy

Selection of an empirical treatment regimen should include consideration of epidemiological features and the clinical course of infection, and consultation with an infectious diseases specialist is recommended, as stated in the guidelines 1.

Diagnosis and Management

The diagnosis and management of endocarditis involve a combination of clinical evaluation, laboratory tests, and imaging studies, with the goal of identifying the causative organism and guiding antimicrobial therapy, as outlined in the European Society of Cardiology guidelines 1.

From the Research

Laboratory Studies for Endocarditis

• The diagnosis of infective endocarditis involves blood cultures, which are considered the most sensitive method for detecting bacteremia 2.
• Variables affecting the isolation of microorganisms causing endocarditis include the volume of blood cultured, the number of blood cultures obtained, prior antimicrobial therapy, the type of microorganism involved, and blood culture technique 3.
• Culture-negative infective endocarditis is most frequently associated with prior antimicrobial therapy and nonbacterial agents, such as fungi, chlamydiae, and rickettsiae 3.
• For the diagnosis of bacterial endocarditis, culture of two to three separately collected blood samples of at least 10 ml, and preferably 20 ml, each generally suffices 3.

Blood Culture Techniques

• Prolonged incubation of blood culture bottles beyond five days may not be necessary for the diagnosis of infectious endocarditis, except in patients with risk factors for Cutibacterium acnes infection 4.
• The use of beta-lactamase, antimicrobial adsorbent resins, or lysis-concentration may be necessary in cases where patients have recently received antibiotics 3.
• At least two blood cultures should be obtained from separate peripheral sites, with each bottle containing ≥7 mL of blood 2.

Antibiotic Therapy

• The choice of antibiotic therapy for bacterial endocarditis is determined by the identity and antibiotic susceptibility of the infecting organism, the type of cardiac valve involved, and characteristics of the patient 5.
• Treatment with aqueous penicillin or ceftriaxone is effective for most infections caused by streptococci, while a combination of penicillin or ampicillin with gentamicin is appropriate for endocarditis caused by enterococci 5.
• Vancomycin should be substituted for penicillin when high-level resistance is present, and resistance of enterococci to multiple antibiotics, including vancomycin, is becoming an increasing problem 5.
• The comparative effects of different antibiotic regimens in terms of cure rates or other relevant clinical outcomes are uncertain, and current evidence does not support or reject any regimen of antibiotic therapy for the treatment of infective endocarditis 6.