Special studies to diagnose IE caused by fastidious bacteria and other organisms must be performed (serological studies). Thereafter, unless clinical or epidemiologic clues suggest an etiological diagnosis, the recommended treatment for culture negative NVE is ampicillin plus gentamicin (standard regimen for enterococcal endocarditis, because in the absence of confounding antibiotic therapy enterococci and staphylococci are unlikely causes of culture-negative NVE, ceftriaxone could be used in this regimen instead of ampicillin. For patients with culture-negative PVE, vancomycin is added to this regimen