Who Needs a Blood Culture, and What Type of Infections Can They Detect?
Key takeaways:
A blood culture tests for bacterial and fungal infections in the bloodstream. But it can’t detect viral infections or an infection somewhere else in the body that has not traveled into the bloodstream.
A blood culture can tell you what type of bug is causing an infection and what the best treatment is.
For a blood culture, a healthcare provider takes a sample of your blood and then sends it to a lab to see if bacteria grows. The blood draw is quick, but the results can take a few days.
A blood culture is a way to test for bacteria that could be causing a bloodstream infection (bacteremia). A blood infection can come from another infection somewhere else in your body. It can also happen when a bacteria is directly introduced into the bloodstream — like from a needle. A blood culture can tell you which specific bacteria is causing the blood infection, and how best to treat it.
What is a blood culture?
A blood culture tests for potential blood infections from bacteria. To perform the test, a healthcare provider takes a sample of your blood and puts it in a petri dish. Then they observe the dish over several days to see if bacteria start to grow.
A blood culture can figure out exactly what kind of bacteria is causing an infection. It can also help figure out which medications are the best to fight the infection. This helps make sure you get the right antibiotic treatment.
Healthcare providers use a blood culture when:
A known infection, like a urinary tract infection, has potentially spread to the bloodstream.
A known infection, like a bone or joint infection, may have started with a blood infection.
There are signs of an infection, like a fever or a high white blood cell count, but it’s not clear if it's an infection or where it is coming from.
There are changes in vital signs that might be from sepsis. Sepsis is a life-threatening condition that occurs when an infection starts to affect the whole body. It can happen with many different types of infection, but it’s more common with blood infections.
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What infections can blood cultures detect?
A blood culture can detect a bacteria that is growing in the bloodstream. You may have heard of specific types of bacteria like Staph aureus or Strep pneumo. A blood culture can also detect fungal infections, like Candida.
But a blood culture can’t test for viruses. It also can’t tell if there’s an infection somewhere else in the body that has not traveled into the bloodstream.
Some infections are more likely to be associated with a blood infection, including:
Endocarditis, an infection of the heart valves
Meningitis, an infection of the lining of the brain
Infections around the spinal cord or in the bones of the spine (vertebral osteomyelitis)
Septic arthritis, an infection in a joint (like the knee or ankle)
An infection of an ICD (defibrillator) or pacemaker in the heart
Infections that come from a catheter in a blood vessel (like a central line or port)
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How are blood cultures collected?
Collecting blood cultures is similar to other blood draws. Healthcare providers collect and test them with extra care to limit the chance of contamination. For a blood culture, a healthcare provider:
Finds a vein where they can draw blood
Puts a tourniquet above that vein
Cleans the skin with an antibacterial solution
Inserts a tiny needle into the vein to draw about 20 mL of blood (a bit more than a tablespoon)
In many cases, a provider will repeat this process with a second vein. This helps make sure the results of your test are accurate and limits the chance of a false-positive result. Usually they’ll draw blood before you take any antibiotics. This also helps improve accuracy and avoids any false negatives.
After the blood draw, they send the sample to the lab for testing.
How long does it take to get results from a blood culture test?
It can take several days to get the full results from a blood culture. This is because a blood culture is tested in stages:
First, a specialist looks at the blood under a microscope using a technique called a “gram stain.” This gives a rough idea about which bacteria is causing the infection.
Then, they put the blood culture on little dishes and give it nutrients to grow. If there’s a lot of bacteria, the blood culture can turn “positive” in a day or so.
The blood is typically cultured for several days and examined daily for different bacteria.
If enough bacteria start to grow, the lab will do more testing to see which antibiotics work best against that specific bacteria.
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What do blood culture results mean?
Blood culture results can be positive or negative:
Negative blood culture: This means there’s a low chance of a blood infection.
Positive blood culture: This means that some type of bacteria grew from the blood sample. This might mean that you have an infection in your blood, or it could be a contaminant.
About 40% of positive blood cultures are false positives. This means bacteria grew in the lab, but there isn’t bacteria growing in your blood. Usually these results are due to bacteria that contaminate the lab equipment. This is why providers take two or three samples.
To figure out the best course of treatment, your provider will also account for your symptoms, other labs, and your history. If they think you have an infection, the blood culture itself is very helpful in choosing the best antibiotic for treatment.
The bottom line
Blood cultures can test for bacterial and fungal bloodstream infections. And they can tell what type of bacteria is causing an infection, which helps healthcare providers figure out the best antibiotic choice for treatment.
Healthcare professionals collect blood cultures with a similar technique as other lab work. But they must use sterile technique and repeat the blood draw two to three times to avoid contamination. And the results can take a little longer than other lab tests. If you have questions about why you need a blood culture or what the results mean, be sure to ask your provider for more details.
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References
Coburn, B., et al. (2012). Does this adult patient with suspected bacteremia require blood cultures? Journal of the American Medical Association.
Doren, G. V., et al. (2020). Practical guidance for clinical microbiology laboratories: A comprehensive update on the problem of blood culture contamination and a discussion of methods for addressing the problem. Clinical Microbiology Reviews.
Pien, B. C., et al. (2020). The clinical and prognostic importance of positive blood cultures in adults. The American Journal of Medicine.
