What lab tests are recommended for a patient suspected of having subacute endocarditis?

Laboratory Tests for Suspected Subacute Endocarditis

Blood cultures are the cornerstone diagnostic test for suspected subacute endocarditis, with at least 3 sets drawn from separate venipuncture sites before starting antibiotics. 1

Subacute endocarditis typically presents with indolent symptoms including low-grade fever, fatigue, anorexia, and weight loss, often with a history of predisposing cardiac conditions or invasive procedures. The diagnostic approach requires a systematic laboratory workup to identify the causative organism and assess disease severity.

First-Line Laboratory Tests

• Blood cultures:

  • At least 3 sets from separate venipuncture sites
  • First and last samples drawn at least 1 hour apart
  • Each set including both aerobic and anaerobic bottles
  • At least 10 ml of blood per bottle
  • Obtain BEFORE starting antibiotics 1

• Inflammatory markers:

  • C-reactive protein (CRP)
  • Erythrocyte sedimentation rate (ESR)
  • Complete blood count with differential (leukocytosis is common)
  • Procalcitonin 2

• Renal function tests:

  • Blood urea nitrogen (BUN)
  • Creatinine
  • Urinalysis (microscopic hematuria may be present) 2, 1

• Cardiac biomarkers:

  • Troponins
  • Creatine kinase (CK) 2

Second-Line Laboratory Tests (for Culture-Negative Cases)

If blood cultures remain negative after 48 hours but clinical suspicion remains high:

• Serological testing for:

  • Coxiella burnetii (Q fever)
  • Bartonella species
  • Brucella species
  • Legionella pneumophila
  • Mycoplasma pneumonia
  • Aspergillus species 1

• Immunological tests:

  • Rheumatoid factor (often positive in subacute endocarditis)
  • Antinuclear antibodies
  • Antiphospholipid antibodies
  • ANCA (can be positive in some cases of subacute endocarditis) 1, 3

• Molecular diagnostic techniques:

  • PCR for Tropheryma whipplei
  • PCR for Bartonella species
  • Fungal PCR
  • Broad-range 16S and 18S rRNA PCR (particularly useful for resected valve tissue) 1, 4

Imaging Studies (Not Laboratory Tests but Essential)

While not laboratory tests per se, these imaging studies are crucial in the diagnostic workup:

• Transthoracic echocardiography (TTE) - first-line imaging
• Transesophageal echocardiography (TEE) - if TTE is negative but suspicion remains high
• Cardiac CT - for evaluating complications like paravalvular abscesses
• 18F-FDG PET/CT - particularly useful for prosthetic valve endocarditis 2, 1

Common Pitfalls and Caveats

• Delayed diagnosis: Don't wait for positive blood cultures if clinical suspicion is high. If cultures remain negative after 48 hours, proceed with serological testing and consider empiric therapy 5

• Inadequate blood sampling: Insufficient volume or improper technique can lead to false-negative cultures. Ensure proper collection of adequate blood volume (10 ml per bottle) 1

• Prior antibiotic use: Can lead to false-negative cultures. Always ask about recent antibiotic use and consider this when interpreting negative cultures 2

• Non-infectious mimics: Some autoimmune conditions can mimic endocarditis. Consider immunological testing if clinical picture is atypical 1, 3

• Mixed infections: Consider the possibility of polymicrobial infection, especially in intravenous drug users 5

By systematically applying these laboratory tests while correlating with clinical findings and imaging results, the diagnosis of subacute endocarditis can be established promptly, allowing for timely and appropriate antimicrobial therapy to improve outcomes.