Infective endocarditis: improving the diagnostic yield.

INTRODUCTION: Isolating aetiological agents in patients with infective endocarditis (IE) remains problematical. We postulated that the high local incidence of culture-negative IE resulted from antibiotic exposure prior to blood cultures and that a structured delay in therapy in the subacute presentation would improve the diagnostic yield.

AIM: We aimed to prospectively observe the diagnostic approach and give an overview of supplementary laboratory tests.

STUDY DESIGN: Patients with suspected IE were enrolled into this analytical observational study and followed up for six months (n = 92). We compared the diagnostic yield and outcome in cases where antibiotics were withheld for 72 hours, with those cases who received early antimicrobials, despite being deemed safe for delayed therapy.

RESULTS: Definitive diagnoses (definite or excluded IE) were made in 92.8% of patients where antibiotics were delayed, compared to 60% of patients who received empirical treatment (p = 0.08). The mortality rates were 18.4% and 30.0% (p = 0.18). Twenty-three of 26 patients with definite culture-negative IE received antibiotics during the 48 hours preceding cultures, compared to eight of 21 culture-positive patients (P < 0.001). Screening for atypical bacteria did not improve the yield. C-reactive protein (CRP) had a sensitivity of 97.0% (negative predictive value 87.5%), whereas a positive rheumatoid factor (RF) had a specificity of 93.8% (positive predictive value 91.7%).

CONCLUSIONS: We observed tendencies towards a greater diagnostic yield and lower mortality where antibiotics were initially withheld. Antibiotic prior to blood cultures were an important cause of culture-negative IE. A normal CRP proved useful in excluding IE; a positive RF strongly favoured IE.