Why Do We Order Two Sets of Blood Cultures?

Let’s answer the question that we have all asked ourselves if we have ever obtained blood cultures before. Why do we need two sets? Why can’t we just obtain one sample. Do we really need the full quantity of blood requested? This is all in response to a hilarious meme by @codebluememes earlier today. Ultimately all the papers cited are free and you can download them for yourself rather than trust me.

Why do we need two independent blood culture samples?

A study from the Mayo Clinic in 2004 showed that two blood cultures only detected 80% of blood stream infections (BSI). Three detected 96% and four were needed to detect 100% of BSI’s. I guess we should be grateful that our policies are only two. Cockerill, et al. found that checking just one blood culture showed positive in 65.1% of patients. When they checked two blood cultures, the percentage bumped up to 80.4%. I guess you should be glad your microbiology lab isn’t asking for three sets because if that’s the case, the percentage increases to 95.7%. All these patients had endocarditis, by the way.

A more recent study by Lee, et al. in 2007 showed that one blood culture caught 73.1% of cases of BSI. If this was increased to two, then the number increased to 89.7%. Three blood cultures detected 98.2% of blood stream infections. That’s a pretty eye opening jump in my opinion.

The studies go further into detail about the microbes they detected and how easy or challenging it was to isolate each microbe. Nicola states in his editorial that in checking only two blood cultures we are still missing 10% of cases of blood stream infections.

Why you can’t cut corners with the amount of blood for blood cultures

The recommendation is to obtain 20cc of blood for each culture. At institutions where I have worked in the past, I have learned that some staff go ahead and obtain just 10cc rather than the 20cc because it’s easier. This is unacceptable and if you do this, here’s why you should stop. Don’t forget that the whole reason you’re being asked to check for blood cultures is for the benefit of the patient. Not to annoy you.

Cockerill, et al. found that there was a relationship between the volume of blood cultured and the detection of bacteremia and bloodstream infection. In their study they sampled 10, 20, 30 and 40cc’s per stick. Talk about iatrogenic anemia. If your patient has a bacteremia and you get 20cc rather than 10cc, you improve your chances of catching the bug by 29.8%. If you obtain 30cc of blood, this improves the yield by 47.2%.

Wrapping up the argument of obtaining two sets of blood cultures

Given how risk averse society is getting these days, it’s actually quite shocking to see that we are only checking two blood cultures in our patients when three would provide a better yield of diagnosing blood steam infections. Please don’t cut corners when checking blood cultures.

If you want to learn about why we pay close attention to the serum bicarbonate/serum carbon dioxide (CO2) on the BMP, CLICK HERE.


Cockerill FR 3rd, Wilson JW, Vetter EA, Goodman KM, Torgerson CA, Harmsen WS, Schleck CD, Ilstrup DM, Washington JA 2nd, Wilson WR. Optimal testing parameters for blood cultures. Clin Infect Dis. 2004 Jun 15;38(12):1724-30. doi: 10.1086/421087. Epub 2004 May 25. PMID: 15227618.
Link to Article

Lee A, Mirrett S, Reller LB, Weinstein MP. Detection of bloodstream infections in adults: how many blood cultures are needed? J Clin Microbiol. 2007 Nov;45(11):3546-8. doi: 10.1128/JCM.01555-07. Epub 2007 Sep 19. PMID: 17881544; PMCID: PMC2168497.
Link to Article

Nicola FG. Assessing how many blood cultures are needed for detecting bloodstream infections. J Clin Microbiol. 2008 Mar;46(3):1155-6; author reply 1156. doi: 10.1128/JCM.02406-07. PMID: 18326835; PMCID: PMC2268350.
Link to Article

Bouza E, Sousa D, Rodríguez-Créixems M, Lechuz JG, Muñoz P. Is the volume of blood cultured still a significant factor in the diagnosis of bloodstream infections? J Clin Microbiol. 2007 Sep;45(9):2765-9. doi: 10.1128/JCM.00140-07. Epub 2007 Jun 13. PMID: 17567782; PMCID: PMC2045273.
Link to Article

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Two Sets of Blood Cultures Reel Transcript

A patient presents to the hospital where an infection is highly suspected.
The guidelines state to obtain blood cultures before antibiotics.
You look at all the bottles in your hand and wonder why do we need to draw so much blood from this poor patient.
I promise it is not to annoy the nurses and phlebotomists.
We know this is technically challenging and causes iatrogenic anemia.
I invited you to read the data for yourself as all these articles are on my website.
Here is why you shouldn’t cut corners when drawing the two sets of blood cultures.
We all wish our tests were 100% but unfortunately they are not.
In a 2004 study in patients in patients with bacteremia
65.1% of bacteremia were detected with one culture.
80.4% with two cultures.
95.7% with three cultures.
Another study in 2007 showed
73.1% of bacteremia detected with one culture.
89.7% with two cultures.
98.2% with three cultures.
Doing two blood cultures still misses 10% of bacteremia
We should not be cutting corners with the volume, either.
I guess we should be happy that the standard isn’t three.