Guide
Blood cultures: contamination, skin antisepsis, and bottle order
6-minute read · Aligned to published WHO phlebotomy guidance
A blood culture is the test that asks whether there are micro-organisms growing in the patient's blood. It is also the test most easily ruined at the bedside. A contaminated culture grows a skin organism the patient never had in their bloodstream, and that false positive can mean days of unnecessary antibiotics, a longer hospital stay, and a treatment decision made on the wrong evidence. The technique exists to keep the result clean, and almost all of it happens before the needle goes in.
Why contamination is the whole problem
Most of what a phlebotomist draws can tolerate a small lapse in skin preparation. A blood culture cannot. The bottles contain a broth designed to grow whatever lands in them, so a single skin organism carried in on an under-prepared site will multiply and report as a positive. Laboratories commonly aim to hold blood-culture contamination to around 3% or below as a local quality benchmark, and the technique below is what keeps a service under it.
Enhanced skin antisepsis
Routine skin antisepsis for a venous draw is a clean swab and a dry. Blood cultures ask for more: an enhanced preparation, with the contact time and the dry time treated as the active ingredients rather than as a delay before the real work.
Many services use chlorhexidine — typically 2% chlorhexidine gluconate in 70% isopropyl alcohol — for this step. Whatever the agent, the rule that protects the result is the same: once the site is cleaned, it is not touched again. Re-palpating the vein after antisepsis puts skin flora straight back onto the prepared site.
The bottle order and the volume
A culture set is two bottles: one aerobic, one anaerobic. Adult fill volumes are set by local laboratory protocol — commonly in the region of 20 mL across the set, around 10 mL per bottle — and volume matters in its own right, because too little blood lowers the chance of detecting an organism that is present. Check the figures your own laboratory specifies.
Two separate questions of order apply, and both follow local protocol rather than the WHO text. Within a multi-tube draw the culture bottles come first of all, ahead of the additive tubes — they sit at the head of the order of draw precisely because they are the most vulnerable to contamination. Within the set, many protocols fill the aerobic bottle before the anaerobic one; follow the order your laboratory specifies.
Drawing through a line
The shape of a clean set
Strip the procedure back and it is short: identify the patient, prepare the site properly and leave it alone, draw the set into the two bottles in order, and label at the bedside. The discipline is not complexity — it is refusing to rush the two timed steps that everyone is tempted to rush.
This guide is a free extract from PhlebMastery's phlebotomy theory course, with content aligned to published WHO guidance. The full treatment — blood cultures alongside arterial, paediatric, capillary, and donor collection — is in Module 7: Special Collections & Advanced Techniques. New here? Start with the free Module 1, or see the whole course — full access is a one-time purchase.
Want the full picture? Read Module 7 in the course, or browse the glossary.
These guides are independent educational materials. They are informed by published WHO phlebotomy guidance and other professional references; they are not WHO materials and are not endorsed or accredited by WHO.