What Is Sensitivity Analysis?
also called susceptibility testing, 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. The results from the test can
help your doctor determine which drugs are likely to be most effective in
treating your infection.
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?
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:
- a persistent sore throat
- a recurring urinary tract infection (UTI)
- an unresponsive case of pneumonia
Sensitivity analysis may be ordered if your infection doesn’t
respond to treatment. This can help your doctor to see if the bacteria that’s
causing your infection has developed resistance. Your 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. Your 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:
- sputum (spit)
- inside the cervix
- a pus-containing wound
Your doctor will send the sample to a laboratory, where it will
be spread on a special growing surface. 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
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
- Resistant means
the bacteria can grow even if the drug is present. This is a sign of an
- 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. 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
- 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 talk to you about potential risks associated
with your sample.
What Are the Results for a Sensitivity Analysis?
Once the bacterial cultures have been grown and tested with
antibiotics, your doctor can analyze the results. These results can help determine
the best antibiotic to treat your infection.
Your doctor will usually choose an
appropriate drug from the report that was listed as “susceptible,” meaning it
can fight the bacteria.
You will be prescribed a drug from the “intermediate” group If
there are no known drugs available in the susceptible group. You will likely
have to take a higher dosage and for a longer time period If you are taking a
drug from the intermediate group. You may also experience medication side
An antibiotic that bacteria, fungi, or another microorganism has
shown 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.
You may be prescribed a combination of antibiotics if a bacterium
is “resistant” to all of the drugs that are usually used to treat an infection.
This combination of drugs is meant to work together to fight the bacteria. Drugs
in this category can be more expensive and may have to be given intravenously
(through a needle in your vein). You will also likely have to take the
combination of drugs for an extended time period.
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
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.