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Blood Culture

More than 200, 000 cases of septicemia occur annually in the US and are associated with a 20-50% mortality rate. Detection of bacteremia is important in establishing the primary diagnosis in high risk patients, confirming the bacterial etiology of a focal infection, detecting complications of focal infections, monitoring antibiotic therapy, and excluding serious infections such as endocarditis.

In 2007, the positive rate in a large tertiary care hospital in the Midwest was 10%. The majority of positive blood cultures yielded Gram-positive bacteria (73%), followed by Gram-negative bacteria (20%), anaerobic bacteria (5%), and fungus (2%). Generally, the following isolates nearly always represent true bacteremia or fungemia when isolated from blood cultures, even if only one culture is positive: Staphylococcus aureus, Streptococcus pneumoniae, Pseudomonas aeruginosa, Candida species, and enteric gram-negative bacteria such as E. coli. Other less commonly isolated organisms that are almost always pathogens include Streptococcus pyogenes, Streptococcus agalactiae, Haemophilus influenzae, Bacteroides fragilis, and Neisseria meningitidis. Enterococci are found to be significant 80% of the time.

The primary organisms responsible for blood culture contamination are skin flora. Coagulase-negative staphylococci are found to be contaminants 60-80% of the time. Other common potential contaminants include viridans streptococci, Corynebacterium species, Propionibacterium, Bacillus species, and Micrococcus. The overall blood culture contamination rate should be below 2.5%.

Other guidelines which are useful in identifying false-positive blood cultures include:

  1. Skin flora is usually a contaminant (diphtheroids, Staphylococcus epidermidis, Bacillus sp).
  2. Enterobacteriaceae, gram negative anaerobes, Streptococcus pyogenes, or Streptococcus pneumoniae are rarely contaminants.
  3. Contaminants are rarely isolated in subsequent cultures.
  4. Multiple organisms isolated from one culture suggest contamination.
  5. Delayed detection of bacterial growth (5 days) is more common for contaminants.
  6. The patient does not have leukocytosis or a left shift.
  7. Primary infection has a different flora.

Culture types

  1. A routine adult blood culture consists of inoculation of one aerobic and one anaerobic bottle per set. Outpatient specimens should be submitted as 2 SPS tubes.
  2. Pediatric blood cultures consist of one aerobic bottle only. Anaerobes should be ordered separately if necessary.
  3. The number of cultures should be specified (either 2 or 3) and will be drawn at one hour intervals unless otherwise specified.
  4. ARD bottles should be requested specifically, if desired.
  5. Fungal, AFB, and viral blood cultures should be ordered separately.

General Guidelines for ordering

The optimal time to collect a blood culture is 30 minutes before the patient spikes a fever. Since this time cannot reliably be predicted, blood culture specimens are appropriately collected during fever and chills, or whenever bacteremia is suspected. Blood cultures are routinely drawn at 60 minute intervals unless otherwise specified. In a critically ill patient who requires immediate antimicrobial therapy, 2 or 3 blood cultures may be drawn simultaneously, optimally from separate sites. Ideally, blood cultures should not be drawn simultaneously with starting an IV line, since this increases the likelihood of contamination.

A routine adult blood culture consists of inoculation of one aerobic and one anaerobic bottle per set. Outpatient specimens should be submitted as 2 SPS tubes. Pediatric blood cultures consist of one aerobic bottle only. Anaerobes should be ordered separately if necessary. Fungal and AFB blood cultures should be ordered separately.

Two or three specimens should be drawn per septic episode or in a 24 hour period. ARD bottles should be requested specifically, if desired. No more than three specimens should be drawn in that time period without consultation with an infectious disease specialist or a clinical pathologist. Studies have shown that 90% of true bacteremia is detected with the first blood culture, and 99% with the first two cultures. Additional cultures increase the chance of isolating a contaminant, which can add significant cost to a hospitalization and increase length of stay. Single blood cultures from adults are inappropriate, since most bacteremias occur intermittently. Single cultures are also difficult or impossible to interpret as contaminants vs. pathogens.

Volume of blood per culture remains the single most important factor in obtaining a pathogen. Optimal volume for adults is 20 mL of blood per venipuncture. Because infants and children have higher levels of bacteremia than adults, 1-5 mL of blood per venipuncture is usually sufficient for organism recovery. Pediatric media has the advantage of requiring only 0.5-5mL blood per bottle. Pediatric media is also suitable for adult patients from whom a lower specimen volume is necessary, or from whom it may be difficult to obtain higher volume venipuncture specimens, such as geriatric patients. Pediatric media is available as aerobic only.

Reference value is negative or no growth.

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