The Relevance of a Culture and Sensitivity Test

A laboratory test is almost always necessary to confirm the presence of infection in the urine, blood, stool, sputum, wounds or other internal body linings like cervix, throat, vagina etc. hence, your doctor may ask you to go for a culture and sensitivity test.

There is a difference between a culture and a sensitivity test, although they are mostly carried out together because you need a culture test in order to carry out a sensitivity test. A culture is a test to check for germs or bacteria in any given sample. For example, your urine sample can be cultured to observe the microorganism that can cause a urinary tract infection (UTI). It involves using certain prepared growing surface called “media” to observe whether any microorganisms will grow on it after a sample of your urine is smeared on it.

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Sensitivity test or analysis, also called susceptibility testing, on the other hand, helps your doctor find the most effective antibiotic to kill an infecting microorganism. Infecting microorganisms are organisms such as bacteria or fungi that invade your body and cause an infection. A sensitivity analysis is a test that determines the “sensitivity” of bacteria to an antibiotic. It also determines the ability of the drug to kill the bacteria. Doctors use sensitivity testing to determine the right antibiotic treatment for an infection and to monitor changes in bacterial resistance to antibiotics. Both are key to your care.

Why Is Sensitivity Analysis Done?

It is possible for bacteria and other pathogens to mutate. Antibiotics that work today may not work six months from now. Sensitivity tests are extremely important and useful tools, especially if you have an infection caused by bacteria that has become resistant to some treatments.

Many bacteria are resistant to common antibiotics. This means that the drug can’t kill the bacteria. Sensitivity analysis is a useful tool to help quickly determine if bacteria are resistant to certain drugs.

Examples of antibiotic-resistant infections include:

Sensitivity analysis may be ordered if the  infection doesn’t respond to treatment. This can help the  doctor to see if the bacteria that’s causing the  infection has developed resistance. The  doctor can then determine which drug would be more effective in treating the infection.

How Is Sensitivity Analysis Performed?

Sensitivity analysis starts with a bacterial sample. The  doctor will get this sample by swabbing the infected area. Your doctor can sample any area that has an infection. Samples may be taken from:

  • blood
  • urine
  • sputum (spit)
  • inside the cervix
  • a pus-containing wound

The  doctor will send the sample to a laboratory, where it will be spread on a special growing plate surface  called “media”. The grown bacteria is known as a culture and bacteria in the culture will grow and multiply. The bacteria will form colonies, or large groups of bacteria, that will each be exposed to different antibiotics.

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These colonies can be susceptible, resistant, or intermediate in response to the antibiotics:

  • Susceptible means they can’t grow if the drug is present. This means the antibiotic is effective against the bacteria.
  • Resistant means the bacteria can grow even if the drug is present. This is a sign of an ineffective antibiotic.
  • Intermediate means a higher dose of the antibiotic is needed to prevent growth.

What Are the Risks of a Sensitivity Analysis?

Few risks are associated with this test especially when it involves using blood samples. Blood collection comes with small risks. For example, you may feel slight pain or a mild pinching sensation during the blood draw. You may feel throbbing after the needle is withdrawn.

Rare risks of taking a blood sample include:

  • Light-headedness or fainting.
  • Hematoma (a bruise where blood accumulates under the skin).
  • Infection (usually prevented by the skin being cleaned before the needle is inserted).
  •  Excessive bleeding (bleeding for a long period afterwards may indicate a more serious bleeding condition and should be reported to the  doctor). The  doctor will talk  about potential risks associated with the  sample.

Further Testing

Some infections may require further testing because it’s known that the drugs normally used to treat the bacteria or fungi causing the infection aren’t always effective. It’s also possible for the sample taken from the infection to have more than one microorganism. Susceptibility testing may be used to figure out which antibiotic or combination of antibiotics will be most effective in treating the different types of bacteria causing the infection.

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