Timing of blood collection
Whenever possible, blood should be taken before antibiotics are administered. The best time is when the patient is expected to have chills or a temperature spike. It is recommended that two or preferably three blood cultures be obtained, separated by intervals of approximately I hour (or less if treatment cannot be delayed). More than three blood cultures are rarely indicated. The advantages of repeated cultures are as follows:
• the chance of missing a transient bacteraemia is reduced;
• the pathogenic role of “saprophytic” isolates (e.g., Staphylococcus epidermidis) is confirmed if they are recovered from multiple venepunctures.
Empirical antibiotic therapy should be initiated after blood specimens for culture have been collected. If necessary, the choice of antibiotic can be adjusted when the results of susceptibility tests become available.
Quantity of blood
Because the number of bacteria per millilitre of blood is usually low, it is important to take a reasonable quantity of blood: 10 ml per venepuncture for adults; 2 - 5 ml may suffice for children; for infants and neonates, 1 - 2 ml is often the most that can be obtained.
The skin at the venepuncture site must be meticulously prepared using a bactericidal disinfectant: tincture of iodine, polyvidone-iodine 10%, alcohol 70%, or chlorhexidine 0.5% in alcohol 70%. The disinfectant should be allowed to evaporate on the skin surface before blood is withdrawn. If tincture of iodine is used, it should be wiped off with 70% alcohol to avoid possible skin irritation.
Even after careful skin preparation, some bacteria persist in the deeper skin layers and may gain access to the blood, e.g., S.
epidermidis, Propionibacterium acnes, and even spores of Clostridium. Pseudobacteraemia (false-positive blood culture) may result from the use of contaminated antiseptic solutions, syringes, or needles. The repeated isolation of an unusual organism (e.g., Pseudomonas cepacia, Enterobacter agglomerans, or Serratia spp) in the same hospital must raise suspicion of a nosocomial infection and promote an investigation. Another source of contamination is contact of the needle with non-sterile vials (or solutions), if the same syringe is first used to provide blood for chemical analysis or measurement of erythrocyte sedimentation rate.
The use of sodium polyanethol sulfonate (SPS) as an anticoagulant is recommended because it also inhibits the antibacterial effect of serum and phagocytes. If the blood is immediately added to a sufficient volume (50 ml) of broth and thoroughly mixed to prevent clotting, no anticoagulant is needed. It is recommended that blood-culture bottles be available at all hospitals and major health centres. If blood-culture bottles are not available, blood may be transported to the laboratory in a tube containing a sterile anticoagulant solution (citrate, heparin, or SPS). Upon receipt in the laboratory, such blood samples must be transferred immediately to blood-culture bottles using a strict aseptic technique. Where blood is taken without anticoagulant, the clot can be aseptically transferred to broth in the laboratory and the serum used for certain serological tests (e.g., Widal).