A contaminated blood culture is not just an inconvenience, it is a patient safety event. A false-positive result from a skin-flora contaminant (Staphylococcus epidermidis, Corynebacterium, Cutibacterium acnes) can lead to unnecessary antibiotic therapy, extended hospitalization, additional procedures, and significant cost. The phlebotomist is the first and most important line of defense against contamination. This protocol is how you do it right.
When Blood Cultures Are Ordered
Blood cultures are ordered when bacteremia or fungemia is suspected, fever, chills, rigors, hypotension, elevated WBC with left shift, clinical sepsis, or before starting antibiotic therapy in a patient with suspected infection. The timing of collection relative to fever spikes is important: ideally collect during or just after a fever spike when bacterial load is highest, though in practice the order should not be significantly delayed.
How Many Sets, and Why
The current standard is two blood culture sets, collected from two separate venipuncture sites. Each set consists of one aerobic bottle and one anaerobic bottle. This gives you:
- Sensitivity improvement: Two sets from separate sites detects approximately 90% of bacteremia. One set detects only ~65-70%.
- Contamination discrimination: If a contaminant (skin flora) is present, it typically grows in only one set. A true pathogen grows in both sets. This distinction is clinically critical for interpretation.
Never collect both sets from the same site. Never draw through an existing IV line unless the physician specifically orders it (and you are following your institution's catheter-draw protocol).
Equipment You Need
- Blood culture bottles: aerobic (blue/gray cap) and anaerobic (purple/orange cap), check your institution's brand
- Chlorhexidine gluconate (0.5% or 2%) swabs or applicators, the gold standard for blood culture prep
- 70% isopropyl alcohol swabs
- Sterile gloves (required for blood culture collection in many facilities)
- 20G or 21G needle (winged infusion set or standard)
- Tourniquet, gauze, bandage
The Protocol: Step by Step
Step 1: Prepare the Bottles
Remove the plastic caps from both culture bottles and clean the rubber septum with 70% isopropyl alcohol. Allow to dry completely (minimum 30-60 seconds). Do not touch the cleaned septum. Set bottles aside. Write the collection time, patient identifiers, and "Set 1 Site 1" on bottle labels if your institution uses paper labeling (electronic systems may differ).
Step 2: Select and Prepare the Venipuncture Site
Choose your site, preferably the antecubital fossa. Apply the tourniquet and palpate to identify the vein before beginning skin prep. Once you begin the antiseptic prep, do not re-palpate without re-cleaning.
Step 3: Two-Stage Skin Antisepsis
This is where most contamination errors occur. The two-stage prep is not optional:
- Stage 1, Alcohol: Clean the site with 70% isopropyl alcohol using a circular outward motion (center to periphery, 4-5 cm diameter). Allow to dry fully, approximately 30 seconds. This removes gross contamination and surface sebum.
- Stage 2, Chlorhexidine: Apply chlorhexidine gluconate using the back-and-forth friction scrub technique (not the circular method). Scrub for a full 30 seconds. Allow to dry completely, a minimum of 30 seconds, preferably 60. Chlorhexidine requires contact time to kill organisms; wiping it off early negates its effect.
Do not blow on the site to dry it faster. Do not fan it with your hand. Let it air dry.
Step 4: Perform Venipuncture
Do not re-palpate the cleaned site with an uncleaned finger. If you must re-palpate, use a sterile-gloved finger or re-prep the site. Insert the needle, confirm blood return, and draw the required volume.
Step 5: Volume Requirements
Volume is the single most important factor in blood culture sensitivity. More blood = better detection:
- Adults: 8-10 mL per bottle, 16-20 mL per set (two bottles)
- Pediatrics: Volume is weight-based; typically 1-3 mL per bottle. Consult your institution's pediatric protocol.
- Minimum acceptable: Most labs reject bottles with less than 3 mL. Check your lab's threshold.
Step 6: Bottle Fill Order
If using a butterfly set: fill the aerobic bottle first. Aerobic bottles have no vacuum inhibitors; the residual air in the butterfly tubing actually benefits aerobic organisms. Fill the anaerobic bottle second.
If using a syringe: fill the anaerobic bottle first (air introduced from the syringe plunger harms anaerobes if they are exposed to it last).
This is a commonly tested distinction on the ASCP PBT exam.
Step 7: Transport
Transport bottles to the lab or incubator as soon as possible, within two hours. Blood culture incubators are maintained at 35-37°C to replicate body temperature and encourage bacterial growth. Do not refrigerate blood cultures. Do not leave them in a cold drawer or car.
The Most Common Contamination Sources (and How to Prevent Each)
When a Culture Grows: True Positive vs. Contaminant
This is the physician's interpretation, but a knowledgeable phlebotomist understands the downstream impact of their technique. Organisms that commonly indicate true infection: Staphylococcus aureus, gram-negative rods (E. coli, Klebsiella, Pseudomonas), Streptococcus pneumoniae, Candida species. Organisms that commonly indicate contamination when only one set grows: coagulase-negative staphylococci, Corynebacterium, Bacillus species, Cutibacterium acnes.
When both sets grow the same organism, the clinical team can treat it as significant regardless of species. When only one set grows, they must weigh clinical context. Your technique is what gives them the data quality to make that decision correctly.