Microbiologic Diagnosis Blood Cultures

Blood culture remains the single most important investigation in a patient suspected of having infective endocarditis. If appropriately collected prior to antibiotic administration, blood cultures can be expected to yield growth of the causative organism in over 90% of cases of infective endocarditis. Identification of the organism may allow the treating physician to determine the original source of bacteremia, and facilitates the choice of the appropriate therapeutic agent(s) and treatment duration.

The Modified Duke Criteria include blood culture as one of the major diagnostic criteria. In order to fulfill the major microbiologic criterion, blood culture support for the diagnosis of IE is defined as isolation of "typical" microorganisms (viridans streptococci, Streptococcus bovis, HACEK group, S. aureus, community-acquired Enterococcus spp.) from at least two separate blood cultures, blood cultures persistently positive for "microorganisms consistent with IE," or a single culture positive for Coxiella burnetti. Rognon et al. [31] retrospectively applied the Duke criteria to 179 IE cases over a 10-year period, and found blood culture to be the most important criterion in establishing a diagnosis of definite IE. Over half of 52 pathology-proven IE cases in this series that were classified as "definite IE" using the Duke criteria before pathology results were available would have been designated as "possible IE" or "rejected" in the absence of blood culture data.

Intravascular infections including IE are characterized by the presence of continuous bacteremia, and in the majority of IE cases most or all of the pre-therapy blood cultures will be positive. Demonstration of continuous bac-teremia by definition requires more than one blood culture result, and the yield of blood cultures is dependent on both the number of cultures obtained and the volume of blood cultured. The effects of blood draw volume and timing on culture yield were investigated by Li et al., who analyzed data from all blood cultures drawn on patients in the Veterans Administration Medical Center in Seattle over an 18-month period [32]. For the majority of patients, one blood culture set consisted of 20 mL divided equally between one aerobic and one anaerobic bottle. The investigators found that a second 20 mL blood draw increased blood culture yield by 17-20%, and that this additional pick-up rate was the same whether the second culture set was drawn immediately after the first, or at any other time within the next 24 hours. The addition of a third 20 mL draw within 24 hours further increased the blood culture yield by 10%. Most experts agree that three separate blood culture sets (20-30 L in two or three bottles) should be sufficient to detect over 95% of IE-associated bacteremias in the absence of preceding antibiotics [33]. In addition to maximizing the diagnostic yield, the practice of obtaining multiple blood cultures can also be useful in determining whether a positive result represents contamination, in which case only one culture would be expected to grow the contaminating organism.

The timing of blood culture draws depends on the overall clinical status of the patient. In the setting of a septic patient with suspected acute IE, therapy should not be delayed to allow blood cultures to be drawn, and two or three separate venipunctures can be performed a few minutes apart while arrangements are made for initiation of empiric antibiotic therapy. This approach is supported by the data reported by Li et al. (see above), who found that the rate of additional positive cultures from a second blood culture set was independent of its timing. Conversely, a clinically stable patient who has been ill for weeks can safely remain off antibiotics for at least 24 hours while serial blood cultures are obtained. In patients who have received antibiotic therapy before being worked up for IE, blood culture media containing antibiotic-inactivating resin should be used, and in selected circumstances withdrawal of antibiotics in order to allow cultures to be drawn would be appropriate.

Newer blood culture media and modern automated blood culture systems represent a significant improvement over older methods. The majority of non-fastidious organisms will trigger a positive signal in blood culture instruments within 72 hours.

Most clinical laboratories incubate routine blood cultures for five days, as most positive cultures appearing after longer incubation represent contaminants. However, some fastidious organisms that cause IE, including the HACEK group, Brucella species and others, may require longer periods of incubation before triggering automated blood culture systems. The majority of fastidious organisms causing IE will grow within ten days, but others (e.g., Bartonella species) can require several weeks to grow and may not trigger blood culture instruments even when they do grow. In the setting of clinically suspected IE, therefore, blood culture specimens require special management within the laboratory. Approaches vary among institutions and include extended incubation of the bottles collected from patients identified as suspect IE cases, terminal subcultures of negative blood culture bottles to solid culture media at the end of the planned incubation period, or a combination of both. Highly specialized culture techniques can be used for isolation of specific rare causes of IE such as Coxiella burnetti, Bartonella species, and Tropheryma whipplei when they are suspected; these methods and pertinent biosafety considerations have recently been reviewed by Houpikian and Raoult [34].

Candida species cause approximately 50% of proven cases of fungal endocarditis. Although blood cultures are thought to have poor sensitivity for detection of candidemia, more specialized blood culture media have no advantage over standard blood culture bottles for detection of Candida species. Special fungal blood culture media such as Bactec Myco-F-lytic bottles are superior in supporting growth of filamentous fungi such as Aspergillus species, and could be considered for use in immunocompromised patients or known IV drug users with suspected IE. The lysis-centrifugation (Isolator) method is superior to other available processes for detection of Histoplasma capsulatum from blood samples. Emboli leading to operative intervention are seen relatively commonly in cases of fungal endocarditis given the typically large vegetation size. Because blood cultures are frequently negative in fungal endocarditis, these emboli can provide crucial information about the causative organism, and they should be cultured and stained for fungal organisms when they are encountered and removed.

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Responses

  • itala
    What culture media is used in infective endocarditis?
    1 year ago

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