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# Blood cultures: technique that avoids contamination

- Procedure

Take them before antibiotics, at least two sets from separate venipunctures, on disinfected, dry skin, with disinfected bottle tops.

Updated Nov 5, 2026

## Key points

- Take blood cultures before starting the antibiotic, without delaying it in a seriously ill patient.
- At least two sets, each from a separate venipuncture, ideally at different sites; each set is an aerobic and an anaerobic bottle.
- Skin: an alcohol-containing antiseptic, respecting contact time and letting it dry completely before puncture.
- Disinfect the bottle tops before inoculating them.
- Prefer a peripheral vein; draw from a line only if line infection is suspected, and then a peripheral set too.

## What it is

Blood cultures look for bacteria or fungi in the blood, above all when sepsis or endocarditis is suspected. Almost all contamination happens during collection: germs from the patient's skin or from the hub of a catheter grow in the bottle and mimic an infection. A false positive leads to unnecessary antibiotics, removal of lines and longer stays; a sample with too little blood can miss the infection. So the CDC stresses skin antisepsis with drying time, disinfecting the bottle tops, preferring peripheral venipuncture and taking at least two sets.

## Indications

- Suspected sepsis or septic shock.
- Suspected infective endocarditis.
- Fever with suspected bacteraemia, at the medical team's judgement.
- Suspected venous catheter-related infection (peripheral and catheter samples).

## Contraindications and precautions

- Do not delay antibiotics in a seriously ill patient while waiting for blood cultures: they must be taken promptly.
- Avoid drawing from an existing line unless indicated: such samples are contaminated more often.
- Do not puncture the arm with an infusion.
- Fill to the volume each bottle indicates: too little blood gives false negatives (exact volume per the laboratory).
- If other tubes are drawn at the same puncture, the blood culture bottles go first.

## Equipment

- An aerobic and an anaerobic blood culture bottle per set, in date and labelled.
- Skin antiseptic per protocol (for example, chlorhexidine in alcohol) and alcohol for the bottle tops.
- A collection set with a bottle adaptor or a syringe, gloves and a clean tourniquet.
- Gauze, a plaster, a sharps container and the laboratory request.

## Step by step (and why)

- Check the order and whether antibiotics were already given, and identify the patient — prior antibiotics lower detection.
- Hand hygiene and gloves — hands are a source of contamination.
- Remove the bottle caps and disinfect each top; let them dry — the tops are not sterile on the outside.
- Choose a peripheral vein and apply the tourniquet — a peripheral vein is contaminated less often than a line.
- Apply the antiseptic per protocol and let it dry completely, without palpating again — without drying it does not disinfect.
- Puncture and fill each bottle to the indicated mark, in the order of the protocol — the right volume detects germs better.
- Withdraw the needle, apply pressure and discard into the sharps container without recapping — it avoids needlesticks.
- Label each bottle in front of the patient with site, date and time — it shows which set grew.
- Take the second set from another venipuncture, ideally at another site, and send promptly — two sets separate infection from contamination.

## Complications

- Contamination with skin germs: a false positive, unnecessary antibiotics and a longer stay.
- A false negative from too little blood or collection after antibiotics.
- A single set: it is hard to tell whether a skin germ is infection or contamination.
- Venipuncture complications: haematoma, fainting or nerve injury.

## Next step: when to call for help

- Report if you cannot collect the sets before the scheduled antibiotic time, so the doctor can decide.
- Report any break in technique (the site was touched, a top was contaminated): the set must be repeated.
- Report deterioration (low blood pressure, confusion, fast breathing) while the patient waits.
- Pass on at once any positive result the laboratory reports.

## Test yourself

Think of the answer, then tap the question to see it.

## How many blood culture sets are taken in an adult, and from where?

At least two sets, each from a separate peripheral venipuncture, ideally at different sites; each set is an aerobic and an anaerobic bottle.

## Why must the skin antiseptic be left to dry?

Because it needs its contact time to kill skin germs, which are the main source of contamination.

## What is done to the bottle tops before inoculation?

They are disinfected, because they are not sterile on the outside even though the bottle is sterile inside.

## Why is peripheral venipuncture preferred over an existing line?

Because the catheter hub is easily contaminated; a line sample is taken only if line infection is suspected, together with a peripheral sample.

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## Connections

## Related 3

- [Venipuncture: drawing blood samples](https://durgical.com/en/nursing/venipuncture-drawing-blood-samples/)
- [Hand hygiene: the five moments and the technique](https://durgical.com/en/nursing/hand-hygiene-the-five-moments-and-the-technique/)
- [Clean and sterile technique: medical and surgical asepsis](https://durgical.com/en/nursing/clean-and-sterile-technique-medical-and-surgical-asepsis/)

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Educational content for students; it does not replace your institution's protocols, supervised clinical training or a prescriber's order. Dose-calculation examples use made-up numbers: they are educational, not a clinical order.

Sources: [Blood Culture Contamination: An Overview for Infection Control and Antibiotic Stewardship Programs Working with the Clinical Laboratory](https://www.cdc.gov/antibiotic-use/core-elements/pdfs/fs-bloodculture-508.pdf) (Centers for Disease Control and Prevention (CDC), 2024); [Prevent Adult Blood Culture Contamination: A Quality Tool for Clinical Laboratory Professionals](https://www.cdc.gov/lab-quality/php/prevent-adult-blood-culture-contamination/index.html) (Centers for Disease Control and Prevention (CDC), 2026); [WHO guidelines on drawing blood: best practices in phlebotomy](https://www.who.int/publications/i/item/9789241599221) (World Health Organization, 2010).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/nursing/blood-cultures-technique-that-avoids-contamination/