Blood Culture Collection: Technique, Timing and How to Avoid Contamination

Blood cultures are the key test for diagnosing bloodstream infections and sepsis, and for choosing the right antibiotic. But a contaminated blood culture, usually caused by skin bacteria getting into the bottle, can lead to unnecessary antibiotics, extra tests and longer hospital stays. Many hospitals aim for a contamination rate below 3%. Nurses take a large share of blood cultures, so technique matters.

When to take blood cultures

• Suspected sepsis or bloodstream infection: fever, rigors, low temperature, or signs of sepsis.
• Suspected infection of central lines, heart valves (endocarditis) or bones.
• Fever in neutropenic patients.
• Take cultures before antibiotics where possible, but never delay antibiotics in a septic patient to obtain them. See our Sepsis Six guide.

How many and how much?

• A “set” is usually one aerobic and one anaerobic bottle.
• Commonly two sets from separate sites (or at separate times) for adults, so that a single positive skin organism can be interpreted.
• Volume matters most: the chance of detecting bacteria rises with the amount of blood. For adults, typically 8–10 mL per bottle (check the manufacturer’s fill line). Children’s bottles need less, based on weight.

Step-by-step technique

1. Check the request and patient identity; explain and gain consent.
2. Gather equipment: blood culture bottles, a blood culture collection set (winged needle with holder), chlorhexidine 2% in 70% alcohol skin wipe, separate alcohol wipes for bottle tops, gloves, tourniquet, sharps bin.
3. Check bottles are in date and undamaged; mark the fill line.
4. Hand hygiene; apron and gloves.
5. Remove the bottle caps and clean the tops with an alcohol wipe; allow to dry. (The caps are not sterile underneath.)
6. Apply the tourniquet and select a vein. Clean the skin with chlorhexidine-alcohol for 30 seconds and allow it to dry completely. Don’t touch the site again after cleaning.
7. Perform venepuncture with the winged collection set.
8. Inoculate the aerobic bottle first when using a winged set (the tubing contains air), then the anaerobic bottle. Hold bottles upright to monitor volume.
9. Release the tourniquet, remove the needle, apply pressure, and dispose of sharps immediately.
10. Label bottles at the bedside (without covering barcodes), noting the site and time. Send to the lab promptly; don’t refrigerate.
11. Document in the notes.

Taking cultures from lines

Avoid taking cultures from existing peripheral cannulas, which have high contamination rates. If a central line infection is suspected, take a set from the line (after scrubbing the hub) and a set peripherally at the same time, and label each clearly. See our central line guide.

Common mistakes

• Not letting the skin antiseptic dry.
• Re-palpating the vein after cleaning without sterile gloves.
• Not cleaning bottle tops.
• Under-filling (missed infections) or over-filling bottles.
• Drawing other blood tubes before the culture bottles (risk of contamination).
• Delays in getting bottles to the lab.

Based on UKHSA UK Standards for Microbiology Investigations (blood cultures), WHO guidance on blood sampling and manufacturer instructions. Educational content: follow local policy.