Chapter 118. Infective Endocarditis (Part 6) The roles of bacteremia and echocardiographic findings in the diagnosis of endocarditis are appropriately emphasized in the Duke criteria. The requirement for multiple positive blood cultures over time is consistent with the continuous low-density bacteremia characteristic of endocarditis (≤100 organisms/mL). Among patients with untreated endocarditis who ultimately have a positive blood culture, 95% of all blood cultures are positive; in 98% of these cases, one of the initial two sets of cultures yields the microorganism. The diagnostic criteria attach significance to the species of organism isolated from blood cultures. To fulfill a major criterion, the isolation of an organism that causes both endocarditis and bacteremia in the absence of endocarditis (e.g., S. aureus, enterococci) must take place repeatedly (i.e., persistent bacteremia) and in the absence of a primary focus of infection. Organisms that rarely cause endocarditis but commonly contaminate blood cultures (e.g., diphtheroids, CoNS) must be isolated repeatedly if their isolation is to serve as a major criterion. Blood Cultures Isolation of the causative microorganism from blood cultures is critical not only for diagnosis but also for determination of antimicrobial susceptibility and planning of treatment. In the absence of prior antibiotic therapy, three blood culture sets (with two bottles per set), separated from each other by at least 1 h, should be obtained from different venipuncture sites over 24 h. If the cultures remain negative after 48–72 h, two or three additional blood culture sets should be obtained, and the laboratory should be consulted for advice regarding optimal culture techniques. Empirical antimicrobial therapy should not be administered initially to hemodynamically stable patients with subacute endocarditis, especially those who have received antibiotics within the preceding 2 weeks; thus, if necessary, additional blood culture sets can be obtained without the confounding effect of empirical treatment. Patients with acute endocarditis or with deteriorating hemodynamics who may require urgent surgery should be treated empirically immediately after three sets of blood cultures are obtained over several hours. Non-Blood-Culture Tests Serologic tests can be used to implicate causally some organisms that are difficult to recover by blood culture: Brucella, Bartonella, Legionella, and C. burnetii. Pathogens can also be identified in surgically recovered vegetations or emboli by culture, by microscopic examination with special stains (i.e., the periodic acid–Schiff stain for T. whipplei), and by use of polymerase chain reaction (PCR) to recover unique microbial DNA or 16S rRNA that, when sequenced, allows identification of organisms. Echocardiography Imaging with echocardiography allows anatomic confirmation of infective endocarditis, sizing of vegetations, detection of intracardiac complications, and assessment of cardiac function (Fig. 118-3). Transthoracic echocardiography (TTE) is noninvasive and exceptionally specific; however, it cannot image vegetations <2 mm in diameter, and in 20% of patients it is technically inadequate because of emphysema or body habitus. Thus, TTE detects vegetations in only 65% of patients with definite clinical endocarditis; i.e., it has a sensitivity of 65%. Moreover, TTE is not adequate for evaluating prosthetic valves or detecting intracardiac complications. TEE is safe and significantly more sensitive than TTE. It detects vegetations in >90% of patients with definite endocarditis; nevertheless, false-negative studies are noted in 6–18% of endocarditis patients. TEE is the optimal method for the diagnosis of prosthetic endocarditis or the detection of myocardial abscess, valve perforation, or intracardiac fistulae. Figure 118-3 Imaging of a mitral valve infected with Staphylococcus aureus by low- esophageal four-chamber-view transesophageal echocardiography (TEE). A. Two- dimensional echocardiogram showing a large vegetation with an adjacent echolucent abscess cavity. B. Color- flow Doppler image showing severe mitral regurgitation through both the abscess- fistula and the central valve orifice. A, abscess; A-F, abscess- fistula; L, valve leaflets; LA, left atrium; LV, left ventricle; MR, mitral central valve regurgitation; RV, right ventricle; veg, vegetation. (With permission of Andrew Burger, M.D.) . Chapter 118. Infective Endocarditis (Part 6) The roles of bacteremia and echocardiographic findings in the diagnosis of endocarditis are appropriately emphasized. allows anatomic confirmation of infective endocarditis, sizing of vegetations, detection of intracardiac complications, and assessment of cardiac function (Fig. 118- 3). Transthoracic echocardiography. patients with definite endocarditis; nevertheless, false-negative studies are noted in 6–18% of endocarditis patients. TEE is the optimal method for the diagnosis of prosthetic endocarditis or the