The microbiological diagnosis of infective endocarditis.

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. Culture-negative infective endocarditis is most frequently associated with prior antimicrobial therapy and nonbacterial agents, such as fungi, chlamydiae, and rickettsiae. 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; however, in cases who have recently received antibiotics it may be necessary to culture an additional two to three blood samples or to consider use of beta-lactamase, antimicrobial adsorbent resins, or lysis-concentration, the last of which is also the most effective method for recovering mycobacteria and fungi from blood.