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Venepuncture and Blood Cultures OSCE Guide Quiz

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When taking blood cultures, which action matters most for reducing the risk of a false-positive result from skin contamination?

A Let the cleaned skin dry fully and do not touch the site again before taking blood. ✓
B Fill the culture bottles without disinfecting their rubber tops, because the protective caps keep them sterile
C Use ordinary venepuncture technique, because the culture bottles already contain growth media.
D Feel the vein again with a gloved finger after cleaning the skin to confirm the puncture site.
E Fill the routine blood tubes before the culture bottles to shorten the time the needle is in the vein.

The answer is A: Let the cleaned skin dry fully and do not touch the site again before taking blood.

Explanation

Why A is correct

See Cleaning the Skin. Option A is correct because blood culture samples are incubated, meaning that they are kept warm so that any bacteria multiply. Even a small number of skin organisms carried into the bottle can therefore grow and give a false-positive result, a report of infection in the blood that is not real. The skin is the commonest source of this contamination, so skin preparation is critical. Clean with 2% chlorhexidine in 70% isopropyl alcohol, then allow the skin to dry completely without fanning, wiping or blowing on it, because the antiseptic only kills organisms while it is wet and acting. After that, do not re-palpate the vein over the cleaned area. If you really must feel it again, you have to clean the skin again. This is the core of the aseptic non-touch technique, ANTT, which means touching only sterile equipment and the cleaned site and keeping everything else clear of them.

Incubation: blood cultures are kept warm, so any skin organisms carried in will grow.
Skin preparation: let the chlorhexidine in alcohol dry completely, without fanning or blowing.
No re-palpation: do not touch the cleaned site again, or clean it again if you must.

Why the other options are incorrect

B. Fill the culture bottles without disinfecting their rubber tops, because the protective caps keep them sterile: The caps protect the top, but the rubber septum (the stopper that the needle pierces) is not sterile and should be disinfected and allowed to dry before use.
C. Use ordinary venepuncture technique, because the culture bottles already contain growth media.: Ordinary venepuncture technique is not enough for blood cultures. They need stricter asepsis, because cultured bacteria multiply and even a few skin organisms can give a false positive.
D. Feel the vein again with a gloved finger after cleaning the skin to confirm the puncture site.: Feeling the vein again with a gloved finger over the cleaned skin can reintroduce skin organisms, even through a glove. If you must feel the vein again, clean the skin again first.
E. Fill the routine blood tubes before the culture bottles to shorten the time the needle is in the vein.: Blood cultures should be filled before routine tubes when both are taken from one venepuncture, so that they are the least likely to be contaminated. Filling the routine tubes first would be the wrong way round.

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More questions from this set. Start the quiz to answer them and see each explanation.

1
When taking blood cultures with a winged collection set and a vacuum holder, which bottle should be filled first, and why?
2
Before routine venepuncture, an alert patient states her full name and date of birth. Her wristband agrees, but the blood-test request displays a different date of birth. What is the best next action?

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