When obtaining blood cultures and an arterial blood gas sample, which should be drawn first, and should the aerobic bottle be filled before the anaerobic bottle?

Blood Culture and Arterial Blood Gas Draw Order

Fill the blood culture bottles first, before the arterial blood gas syringe, and yes—fill the aerobic bottle before the anaerobic bottle.

Primary Recommendation: Blood Cultures Take Priority

The blood culture bottles should be filled before the arterial blood gas (ABG) syringe for several critical reasons:

• Blood cultures represent potentially life-threatening infections and require optimal specimen quality without any contamination from other substances 1
• ABG syringes contain heparin (either liquid or dry), which can contaminate blood culture bottles and potentially inhibit bacterial growth if the ABG syringe is filled first 2
• Cross-contamination risk is unidirectional: heparin from an ABG syringe can affect blood cultures, but blood cultures do not affect ABG results 2

Aerobic Bottle Fills First

Yes, the aerobic bottle should be filled before the anaerobic bottle in standard practice:

• When collecting 10 mL of blood or less, it should be inoculated into a single aerobic blood culture bottle 1
• For adults, use a 2-3 bottle blood culture set with at least one aerobic and one anaerobic bottle 1
• The aerobic bottle is prioritized because most clinically significant pathogens grow aerobically, though some gram-positive organisms like Streptococcus pneumoniae may grow best in the anaerobic bottle 1

Practical Algorithm for Draw Order

Step 1: Prepare the site

• Disinfect with 2% chlorhexidine gluconate in 70% isopropyl alcohol (preferred) or tincture of iodine 1
• Allow 30 seconds drying time for chlorhexidine/alcohol or 2 minutes for povidone-iodine 1

Step 2: Perform venipuncture

• Use a single venipuncture when possible to minimize patient discomfort 1

Step 3: Fill blood culture bottles in this exact order

• First: Aerobic blood culture bottle (typically 8-10 mL for adults) 1
• Second: Anaerobic blood culture bottle (typically 8-10 mL for adults) 1
• Wipe injection ports with 70-90% alcohol before injecting blood 1

Step 4: Fill ABG syringe last

• After blood cultures are complete, fill the ABG syringe 3
• This prevents heparin contamination of blood cultures 2

Critical Volume Requirements

• Adults require 20-30 mL total per blood culture set (divided between aerobic and anaerobic bottles) 1
• Volume is more critical than timing for blood culture yield 1
• Inadequate volume is a common pitfall that reduces diagnostic sensitivity 1

Common Pitfalls to Avoid

• Never fill the ABG syringe first: Liquid heparin can decrease PCO2, PO2, HCO3, and base excess values, and more importantly, can contaminate blood culture bottles if any backflow occurs 2
• Do not underfill blood culture bottles: This is the most common error affecting culture yield 1
• Avoid drawing blood cultures through catheters when possible: Catheter-drawn blood cultures have higher contamination rates (false positives) compared to peripheral venipuncture 1
• Do not skip or rush the skin preparation drying time: Inadequate drying dramatically increases contamination rates 1, 4

Special Consideration: Arterial Catheters

If drawing from an indwelling arterial catheter rather than performing arterial puncture for the ABG:

• Arterial catheters show intermediate contamination rates between peripheral venipuncture and central lines 4, 5
• In critically ill patients, arterial catheter blood cultures have 85% sensitivity and 95% specificity for true bloodstream infection 6
• The same principle applies: fill blood culture bottles before the ABG syringe to prevent heparin contamination 5, 6
• Gram-positive organisms in arterial line cultures are more likely to represent contamination, while gram-negative organisms may reflect true bacteremia 5