[punkstar]

Why do we draw blood cultures? How long does it take for results to come back? Why does it matter to draw blood cultures from two different sites?

These are three questions that patients ask me regarding blood cultures. I'm curious as to what other ED RNs answers are towards these questions.

[nuangel1]

we draw blood cultures to see if pt is septic .the final report takes 72 hrs .but the sample might grow out immediately and a preliminary report is made.it helps drs to tayler the antibiotics to what grows out of blood cultures .antibiotics treat different organisms.in my ed we do them on anyone with temp 101 or greater.its important to draw thm 15 min apart because 1st sample may not grow anything out but the second sample may .but f someone has very poor access i will draw both sets at same time.

[Angie O'Plasty, RN]

Blood cultures are drawn when sepsis is suspected. This is a very serious disease that can be fatal, so immediate identification is key to treatment.

So here's the scenario: Pt comes in, appears to have an infection: temp up, WBC count up, discomfort of some kind, possibly having taken a round or two of antibiotics without getting a good result, and now the patient/family are really concerned.

Labs are drawn, blood cultures are drawn. It's critical to draw those blood cultures BEFORE any prophylactic antibiotics are given because obviously, the antibiotic will kill some of the bacteria that may be there.

It can take a couple of days for the results to come in because what blood cultures mean is that they're trying to make any bacteria grow on the culture dish. Of course, if there are no bad bacteria, they cannot grow.

Why take BCs from two different sites? I'll let someone else answer that, because to be perfectly honest, I'm not sure. Perhaps to differentiate the location of the infection, supposing that the infection might be local as opposed to systemic?

[Cat84]

I always thought the two sites was so that, if one draw was positive, the results could be confirmed (if it is a true systemic infection it should be in both samples). Also the reason we do not draw BC off old CL (testing the line, not the true system). Also blood cultures are easy to contaminate wih skin flora. CAT

[Pocahontas98_21]

Blood cultures are done to detect and identify bacteria and yeasts (a type of fungus) in the blood. Some bacteria prefer oxygen (aerobes), while others thrive in a reduced oxygen environment (anaerobes). Blood cultures are usually drawn into vials containing two types of media to detect both types of bacteria.

Multiple samples are collected and different veins are used. This is for two reasons: 1) when multiple samples are done, you have a better chance of detecting the infection; and 2) sometimes, despite disinfection of the skin where the blood is collected, you can grow a skin contaminant, which will result in a positive blood culture that is not clinically significant (a false positive). With multiple samples, you have a better chance of ruling out a false positive and deciding what is a true bacteremia. These multiple samples pose no additional risk to you.

[al7139]

At my hospital, we get lots of elderly pts from LTC or SNF's. If these pts have existing PICC lines, Central Lines, or Udall's we always draw one of the cultures from the line in case the infection has originated there. If the draw from the line comes back positive, the line is removed, and the tip is cultured to confirm the bacteremia. In a pt where they do not have an existing line, the cultures are drawn from 2 sites because like checking stool for C-Diff, you can get false readings. I have seen that generally, if the pt is afebrile, and not displaying signs of septicemia, but infection is suspected, the blood cultures are drawn from 2 sites to rule out false positives, since contamination can occur even with aseptic technique. The only times I see positive cultures as significant to our doctors is if both are positive, and they grow a bacteria that is not considered "normal" to the body, or if the indwelling line tests positive.
As to the time it takes to get results, it varies. I usually see preliminary reports at around 24 hours, which will state "growth" or "no growth" If there is growth, the determination of the type of bacteria, and what its sensitive to can take from 48-72 hours, at which point the MD will continue the same treatment, or change it if needed.
Most positives I have seen from blood are staph bacteria.

[august07]

You always want to draw from 2 different sites because of the risk of contamination. Coag negative staph and diptheroids are common skin contaminents that can sometimes cause false positives. When the lab calls and says there are 1 out of 4 bottles that are positive, that may be an indicator of contamination, whereas 3/4, it's probably a true infection. Also, the sooner you get a call, the more likely it's a true infection.

When blood cultures are received in the lab, they are incubated in a machine that flags them as positive when it detects gases that are emitted by the bacteria. Only then is the blood plated out on a dish and worked up. Blood cultures are incubated this way for 5 days, then get thrown out, unless the doctor is looking for some specific bacteria that takes longer to grow.

Also, if you ever have only a tiny amount of blood that can be drawn, always put it in the aerobic bottle (the blue one) because that's where most positives come from.

[Angie O'Plasty, RN]

Thanks, august07, I learned a lot from that!