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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?

You may need this 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?

Test results may vary depending on your age, gender, health history, the method used for the test, and other things. Your test results may not mean you have a problem. Ask your healthcare provider what your test results mean for you. 

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 is usually done with at least two 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?

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be 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. Be sure your healthcare 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
  • Haldeman-Englert, Chad, MD