Blood Culture

Blood culture and sensitivity test, blood C&S

This blood test finds out if you have a systemic infection. This is an infection that affects your whole body, not just 1 part. A sample of blood is studied in a lab to check for bacteria or a type of fungus called yeast. 

You may need this test if you have symptoms of a systemic infection. These include:

• Chills

• Fever or low temperature

• Severe tiredness (fatigue)

• Confusion

• Nausea

• Fast breathing or heart rate

• Passing urine less often

• Cough

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.

If your healthcare provider believes you have pneumonia, a test called a Gram stain may be 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 to check for infection.

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.

Test results may vary depending on your age, gender, health history, and other things. Your test results may be different depending on the lab used. They 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.

The test is often done with 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 bottles 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.

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. 

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

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. Tell your provider about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illegal drugs you may use.