What is an antibiotic sensitivity test?

Antibiotics are medicines used to fight bacterial and certain fungal infections. They are not effective for viral infections. There are different antibiotics, and each one only works well against certain types of bacteria or fungi. An antibiotic sensitivity test can help find out which antibiotic will be most effective in treating your infection.

The test can also help find a treatment for antibiotic-resistant infections. Antibiotic resistance happens when standard antibiotics become less effective or ineffective against certain bacteria. That means that the bacteria change and resist the effect of the antibiotic. The antibiotic no longer works well against the bacteria causing your infection. This is not the same as antibiotic misuse (not taking all of your medicine or taking someone else's medicine), which can contribute to antibiotic resistance. With antibiotic resistance, some diseases that were once easily treated can become serious or even life-threatening.

Other names: antibiotic susceptibility test, sensitivity testing, antimicrobial susceptibility test, drug resistance testing, culture and sensitivity, C & S

What is it used for?

An antibiotic sensitivity test is used to help find the best treatment for a bacterial infection and certain fungal infections.

Why do I need an antibiotic sensitivity test?

You may need this test if you have a bacterial or fungal infection that has been shown to have antibiotic resistance. These could include tuberculosis, MRSA, and C. diff.

What happens during an antibiotic sensitivity test?

To find out what type of bacteria or fungi you have, your provider may order a fungal or bacteria culture. The test is done by taking a sample from the infected site. The most common types of tests include:

• Blood culture
  • A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial.
• Urine culture
  • You will provide a sterile sample of urine in a cup, as instructed by your health care provider.
• Wound culture
  • Your provider will use a special swab to collect a sample from the site of your wound.
• Sputum culture
  • You may be asked to cough up sputum (a mucus, also called phlegm, made in the lungs) into a special cup, or a special swab may be used to take a sample from your nose.
• Throat culture
  • Your provider will insert a special swab into your mouth to take a sample from the back of the throat and tonsils.

Your sample will be tested against different antibiotics to see if they are effective in killing the germs causing your infection.

Will I need to do anything to prepare for the test?

There are no special preparations needed for an antibiotic sensitivity test.

Are there any risks to the test?

There is very little risk to having a blood culture test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

There is no risk to having a throat culture, but it may cause slight discomfort or gagging.

There is no risk to having a urine, sputum, or wound culture.

What do the results mean?

Your results will show how effective different antibiotics were in killing the bacteria or fungus causing your infection. Results are usually described in one of the following ways:

• Susceptible. The tested antibiotic stopped the growth or killed the bacteria or fungus causing your infection, so it may be a good choice for treatment.
• Intermediate. The antibiotic may work at a higher dose, or you may need to take the dose more often.
• Resistant. The antibiotic did not stop the growth or kill the bacteria or fungus causing the infection. It would not be a good choice for treatment.

If your results are resistant to all of the medicines, you may need to take a few medicines that may work better together.

If you have questions about your results, talk to your provider.

Is there anything else I need to know about an antibiotic sensitivity test?

Incorrect use of antibiotics has played a big role in the rise in antibiotic resistance. Make sure you use antibiotics the right way:

• Take them exactly as your provider tells you. Make sure to finish all of the antibiotics.
• Do not use antibiotics for viral infections. They don't work on viruses like colds and flu.
• Don't save them for later. Talk to your pharmacist about safely discarding leftover medicines.
• Don't share your antibiotics with others.
• Don't take antibiotics prescribed for someone else.

Talk with your provider or pharmacist if you have any questions about your antibiotics.

Learn more about laboratory tests, reference ranges, and understanding results.