Sensitivity Analysis

Written by Christine Case-Lo | Published on July 16, 2012
Medically Reviewed by George Krucik, MD

Sensitivity Analysis

Sensitivity analysis, also called susceptibility testing, helps find the right antibiotic to kill an infecting microorganism. This test determines the “sensitivity” of a colony of bacteria to an antibiotic. It also determines the ability of the drug to kill the bacteria.

Why Is Sensitivity Analysis Done?

Unfortunately, many bacteria are resistant to common antibiotics. Resistant 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. It may also be used if you have a fungal infection.

Examples of antibiotic-resistant infections include:

  • a persistent sore throat
  • a recurring urinary tract infection (UTI)
  • an unresponsive case of pneumonia

How Is Sensitivity Analysis Performed?

Sensitivity analysis starts with a bacterial sample. Your doctor will obtain this sample by swabbing the infected area or secretions of the infected area. Your doctor can sample any area that has an infection.

Cultures may be taken from:

  • blood
  • urine
  • sputum (“spit”)
  • inside the cervix
  • a wound

Your doctor will send the specimen to a laboratory, where it will be spread on a special growing surface or culture. Bacteria in the culture will grow and multiply. The bacteria will form colonies (large groups of bacteria) that will each be exposed to different antibiotics. The colonies show up as susceptible, resistant, or intermediate.

Susceptible means they can’t grow if the drug is present. This indicates an effective antibiotic.

Resistant means they can grow even if the drug is present. This indicates an ineffective antibiotic.

Intermediate means a higher dose of the antibiotic is needed to prevent growth.

Your doctor can use the results to determine the best antibiotic to treat your infection.

What Are the Risks of a Sensitivity Analysis?

Few risks are associated with this test. 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 You may feel throbbing after the needle is withdrawn.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:

  • lightheadedness 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 your doctor)

Your doctor will advise you of potential risks associated with your sample.

What Are the Results for a Sensitivity Analysis?

An antibiotic that bacteria, fungus, or another microorganism shows resistance to shouldn’t be used to treat your infection. Your doctor will decide which drug is best if several antibiotics are shown to be effective in killing the microorganism causing your infection.

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