Special Topic: Infective Endocarditis 

Acute Infective Endocarditis   |   Subacute Infective Endocarditis   |   Fungal Infective Endocarditis   |   How Many Cultures?

Blood culture is essential in the diagnosis of infective endocarditis (infection of the heart valves). In this elusive disease, blood cultures may need to be taken repeatedly during febrile episodes, when bacteria are shed from the heart valves into the bloodstream. For patients with infective endocarditis, positive blood cultures will be obtained in over 90% of cases, if optimal culture conditions are respected.⁴¹

This is a fulminant illness progressing rapidly over days to weeks, which may be caused by highly virulent pathogens, such as Staphylococcus aureus. When suspected, the severity of this disease requires blood cultures to be drawn immediately to avoid unnecessary delays in appropriate treatment.

• Multiple blood culture sets should be drawn during a 30-minute period prior to administration of empiric antimicrobial therapy.⁴²

If subacute infection is suspected, although it is important to attempt to establish the microbiological diagnosis, antimicrobial therapy should nevertheless be initiated as soon as possible.

• Multiple blood culture sets should be obtained prior to initiation of antimicrobial therapy, with sets spaced 30 minutes to one hour apart. This may help document a continuous bacteremia, and could be of additional clinical value.¹

Fungal endocarditis is a relatively rare disease, with fungi responsible for <10% of infective endocarditis cases.⁴³ However, it is the most severe form of infective endocarditis, and is associated with high mortality and morbidity.^44,45 Candida species are the most common fungal pathogens involved in infective endocarditis.⁴⁰ Diagnosis of fungal endocarditis can be very challenging, as blood cultures may take a long time to yield growth, with a yield for positive blood cultures of around 50%.⁴³

In order to distinguish between contamination and true bacteremia, a total of three to five blood culture sets should be sufficient.

• Initially, two to three blood culture sets should be obtained from patients with suspected infective endocarditis. If the results of these sets are negative at 24 hours, two more sets of cultures should be obtained, for a total of five sets overall.¹ 

Often patients with suspected infective endocarditis have been put on antibiotics prior to blood collection. This is the most common reason for “culture-negative” infective endocarditis. It is therefore important to use a blood culture medium that has antimicrobial neutralization capacity in order to sustain microbial growth in the presence of antibiotics (see chapter 10 “Which media to use?”).^24-28

However, “culture-negative” endocarditis may also be due to fastidious microorganisms, such as Aspergillus spp., Brucella spp., Coxiella burnetii, Chlamydia spp. and HACEK* microorganisms.

• Since current continuous-monitoring blood culture systems can recover all HACEK and other fastidious organisms within a 5-day period, extending incubation beyond this period is no longer considered to be necessary. However, if all blood culture bottles are negative after 5 days, and infectious endocarditis is still suspected, all bottles should be subcultured to chocolate agar.⁴⁶

*HACEK = Haemophilus species, Aggregatibacter species, Cardiobacterium hominis, Eikenella corrodens and Kingella species³⁶