• Blood cultures are critical in the diagnosis and management of IE.
• Obtain blood cultures before starting antimicrobial therapy whenever possible.
• Intravascular infection leads to constant bacteremia originating from vegetations. Therefore, it is unnecessary to await the arrival of a fever spike or chills to obtain blood cultures.
• There is no significant diagnostic benefit gained from using arterial versus venous blood for culture.
• Always use strict aseptic technique and optimal skin preparation when collecting blood for culture. The recommended antiseptic skin preparation is liberal swabbing with 70 per cent isopropyl alcohol applied in widening circles over a reasonably wide area of skin around the intended site for venepuncture and allowed to dry followed by similar application of an iodophore or tincture of iodine over the same area.
• Blood cultures should be obtained by way of fresh venepunctures and not through indwelling intravascular devices.
• For suspected cases of acute IE obtain atleast two, preferably three sets of blood cultures within 5 to 10 minutes of each other before starting empiric antibiotic therapy.
• For suspected cases of subacute IE draw three separate blood cultures, spaced 30 minutes to 1 hour apart. If these remain negative at 24 hours, draw two further separate cultures.
• The volume of each blood sample drawn should be 20 ml for adults, 1 to 2 ml for neonates, 2 to 3 ml for infants aged 1 month to 2 years, 3 to 5 ml for older children, and 10 to 20 ml for adolescents.
• Each separate blood culture should be divided for inoculation into two bottles. One anaerobic bottle should be included in the total of four bottles inoculated from the two samples to enhance the recovery of certain facultative anaerobes such as streptococci, especially nutritionally variant streptococci.
• If all blood cultures remain negative at 5 days but IE remains likely on clinical grounds, subculture bottles on to chocolate agar plates if the bottles are not held beyond 5 days.
• For optimal processing, the laboratory should be advised that endocarditis is a possible diagnosis and which, if any unusual bacteria are suspected (Legionella species, Bartonella species, HACEK organisms).