Reference Range

The reference range for blood culture is no growth.

Interpretation

True infection is almost always present if the culture is positive for one of the following organisms:^{[1, 2]}

Streptococci (non-viridans)
Aerobic and facultative gram-negative rods^[3]
Anaerobic cocci
Anaerobic gram-negative rods
Yeast

Negative growth does not rule out infection.

Suspect contamination if only one of several cultures is positive, if detection of bacterial growth is delayed (≥5 d), or if multiple organisms are isolated from one culture.^[4]

Common contaminants include the following:

Staphylococcus epidermidis^[5]
Bacillus species
Propionibacterium acnes
Corynebacterium species
Clostridium perfringens
Viridans Streptococcus
Candida tropicalis

A literature review by Hsieh et al indicated that in blood cultures for patients with bacteremia, a short time to positivity (TTP) is predictive of septic shock and mortality, the odds ratios for these being 4.06 and 2.98, respectively. An exception is the Candida species, with the investigators finding no significant correlation between short TTP and mortality for these bacteria. It has been hypothesized that a short TTP signals the presence of a higher bacterial load and, thus, a greater likelihood of mortality.^[6]

Collection and Panels

Specifics for collection and panels are as follows:

Specimen type - Whole blood
Container - Culture bottles (one aerobic and one anaerobic) for blood and green-top tube (heparin) for fungus and mycobacteria (if warranted by clinical suspicion)
Collection method - Venipuncture
Specimen volume - Adults: 10-20 mL per culture set; Pediatric patients: 1.0-3.0 mL

Other instructions are as follows:

Collect specimens as soon as possible after onset of chills or fever and before beginning antibiotic therapy.
Use aseptic technique (eg, clean venipuncture sites and culture vial tops with 2% chlorhexidine/70% isopropyl alcohol swabs before collection).
Draw 2-3 sets of cultures from separate sites at least 30-60 min apart (no more than 4 sets per 24-hour period).
Do not draw from IV catheter unless other sites unavailable.
Related tests - Complete blood count (CBC), urine culture, bacterial wound culture, gram stain, CSF analysis, fungal tests, susceptibility testing, sputum culture

Description

Blood cultures are used to identify microorganisms in the blood and to assist in guiding antimicrobial therapy. Common sources of bacteremia include the following:^{[1, 7]}