Blood Culture Bottle Collection Order
When drawing blood cultures, inoculate the aerobic bottle first, followed by the anaerobic bottle. This practice minimizes the risk of introducing air into the anaerobic bottle and optimizes recovery of both aerobic and anaerobic organisms 1.
Rationale for Bottle Order
Drawing the aerobic bottle first prevents air contamination of the anaerobic bottle, which could compromise the recovery of obligate anaerobes 1.
The aerobic bottle should receive blood first because most clinically significant bloodstream pathogens are aerobic or facultative organisms, and any air introduced during collection will not affect their recovery 1.
After filling the aerobic bottle, the anaerobic bottle should be inoculated second to maintain optimal anaerobic conditions for organisms requiring reduced oxygen environments 1, 2.
Volume Distribution Strategy
Each bottle should receive 8-10 mL of blood for a total of 20 mL per blood culture set in adults 1.
When insufficient blood volume is available (less than 20 mL), inoculate only the aerobic bottle rather than splitting inadequate volumes between both bottles 1.
For pediatric patients weighing ≤12.7 kg with limited blood volumes (≤10 mL total), inoculate all blood into a single aerobic bottle rather than splitting between aerobic and anaerobic bottles 1, 3.
Clinical Importance of Including Both Bottle Types
Both aerobic and anaerobic bottles should be used routinely in adult blood culture sets because the anaerobic bottle significantly increases recovery of Staphylococcus aureus, anaerobes, and certain Enterobacteriaceae 2, 4.
Studies demonstrate that using one aerobic plus one anaerobic bottle yields significantly more pathogens than using two aerobic bottles (P < 0.001 for organisms requiring single positive set, P = 0.04 for conditional pathogens) 4.
The anaerobic bottle alone detects 15-18% of positive blood cultures that would be missed if only aerobic bottles were used 5.
Obligate anaerobes are recovered significantly more frequently from anaerobic bottles (P = 0.03), though they now represent fewer than 5% of all blood culture isolates 1, 6.
Special Circumstances Favoring Aerobic-Only Strategy
When fungemia due to yeast is strongly suspected, consider splitting blood between two aerobic bottles instead of the standard aerobic-anaerobic pair, as yeasts are highly aerobic organisms 1, 7.
For patients with suspected Candida bloodstream infection, using two aerobic bottles or specialized fungal media (such as MycoF/Lytic) may optimize recovery 1.
Common Pitfalls to Avoid
Never inoculate the anaerobic bottle first, as this increases the risk of introducing air during the collection process, which compromises anaerobic organism recovery 1.
Avoid underfilling bottles—inadequate blood volume is the most common reason for false-negative blood cultures, as bacteremia concentration is often less than 1 organism per milliliter 1, 7.
Do not assume anaerobic bottles are unnecessary based solely on low anaerobic bacteremia rates; the anaerobic bottle enhances recovery of facultative organisms like S. aureus beyond just obligate anaerobes 2, 4.
Ensure proper skin antisepsis with 2% chlorhexidine in 70% alcohol or iodine-containing solution applied twice, allowing 30 seconds drying time to minimize contamination rates below 3% 1, 7.