Health Encyclopedia

Blood Culture

Does this test have other names?

Blood culture and sensitivity test, blood C&S

What is this test?

This blood test finds out whether you have a systemic infection. This is an infection that affects your whole body, not just one part. A sample of blood is studied in a lab to check for bacteria or a type of fungus called yeast. 

Why do I need this test?

Your healthcare provider may do a blood culture and sensitivity test if you have symptoms of a systemic infection. These include:

  • Chills

  • Fever

  • Tiredness (fatigue)

  • Confusion

  • Nausea

  • Rapid breathing or heart rate

  • Passing urine less often

A blood culture and sensitivity test can be done to confirm an infection, such as pneumonia, and figure out the best way to treat it.

What other tests might I have along with this test?

If your healthcare provider suspects you have pneumonia, he or she might also order a test called a Gram stain done on a sample of sputum, which is mucus that you cough up. This helps find out what is causing the infection.

You may also have susceptibility testing. This test finds out which antibiotic can treat your infection. You may also have a complete blood count (CBC) either before or with the blood culture. The CBC test shows if you have a higher level of white blood cells, which can also be a sign of infection. Your urine may also be tested.

The blood culture test may need to be repeated if it comes back negative but you still have symptoms. It may also be repeated after you take antibiotics to make sure the infection is gone.

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your healthcare provider.

A positive result means bacteria or yeast are present in your blood. A negative result means that no signs of any bacteria or yeast were found in the blood.

How is this test done?

The test usually requires at least 2 blood samples, drawn through a needle from different veins. Taking multiple samples is more likely to produce accurate results. The blood samples are placed in a dish with a substance that promotes growth of bacteria or yeast. This is called a culture.

Early results may be available in 24 hours, but it can take 48 to 72 hours to find out the specific bacteria or yeast causing your infection.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

Antibiotic medicine could slow the growth of the infecting bacteria. If possible, the blood sample should be drawn just before you begin taking antibiotics.

How do I get ready for this test?

You don't need to prepare for this test. If you are taking antibiotics, let your healthcare provider know the time of your last dose. Also be sure your provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.

Medical Reviewers:

  • Fraser, Marianne, MSN, RN
  • Sather, Rita, RN