How do you read a TB skin test?

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Nicole Galan
by Jill Seladi-Schulman, PhD
to expect Results Takeaway
The purified protein derivative test confirms
whether someone has had exposure to tuberculosis, a serious infectious disease.
A positive test result indicates that the person may have a tuberculosis
infection, but it does not necessarily mean that ethey have active tuberculosis.
Tuberculosis, or TB, is a bacterial infection
that can affect any part of the body but most commonly affects the lungs. There
are two types of TB infection:
Latent infection: In the latent form of the
disease, the bacteria are dormant, which means that they are not active.
Someone with this condition does not feel unwell or have any symptoms. A latent
infection cannot spread to other people.

Active infection: Someone with an active
infection has TB. The amount of time that it takes to progress from the latent
infection to active disease varies. The symptoms of active TB depend on which
part of the body it affects, but they can include unexplained weight loss,
fatigue, loss of appetite, fever, and night sweats.
To carry out a purified protein derivate
(PPD) test, a healthcare professional will put a small amount of the protein
that occurs in the TB-causing bacteria under the person’s skin. If they have
had exposure to the disease, the protein will react with the immune system, and
a red bump will appear at the injection site within a few days.
to expect
A PPD test is quick and relatively painless.
A nurse or another healthcare professional
will first administer an injection with the TB protein. They will use a tiny
needle to place the liquid directly under the skin so that a small blister
forms. The injection may cause a little discomfort, but the liquid should not
burn or feel uncomfortable.
Sometimes, the person administering the
injection will mark the site by circling the area with a pen. They do this to
make the test results easier to read. The injection site does not require a
dressing, and the skin should not have any creams or lotions on it.
It is also important to avoid itching or
rubbing the injection site. If it becomes uncomfortable, a cold compress can
provide relief.
Once back at home, people can resume their
normal life and hobbies, including showering, swimming, and other physical
A healthcare professional will need to
determine the results within 2–3 days of the test. They will check the
injection site for any signs of an immune reaction.
If a person receives the injection but does
not return for their results within the specific timeframe, they will need to
have the test again.
It is unlikely that someone having this test
will experience any side effects, although swelling, blistering, or ulcers are
possible at the injection site. If any of these side effects occur, they should
resolve within a few weeks with no long-term scarring or complications.
A healthcare professional will measure the
induration, which is the raised, swollen, red area. A small area of induration
denotes a negative result. Such an area will usually be 5–15 millimeters in
diameter, depending on the person’s risk of potential exposure to TB. A larger
area of induration signifies a positive result.
It is important to realize that a positive
PPD test result does not mean that someone has TB. However, the individual will
require further testing to determine whether they have a latent or active
It is also possible to have a false positive
or false negative reaction to the test. There are many reasons why this could
happen. A false negative can occur if:
  • Exposure to the infection is very recent.
  • The infection is from a very long time ago.
  • Someone has had a recent vaccination using a
    live virus.
  • The healthcare professional performed the
    test incorrectly.
A false positive can occur if:
  • The healthcare professional either carried
    out the test incorrectly or misread the results.
  • Someone has an infection with a different
    strain of the bacteria.
  • Someone has had treatment with a BCG vaccine,
    which is a standard treatment for early bladder cancer.
A doctor will help interpret the test results
and provide guidance on the next steps.
The PPD skin test is a simple test that can
help screen for tuberculosis, a severe infectious disease.
By injecting a small sample of the
tuberculosis protein under the top layer of the skin, healthcare professionals
can determine whether someone has had exposure to the bacteria that cause the
People should speak with their doctor
regarding any concerns that they have about their risk for TB or their test
Kayode Ojo
Kayode Emmanuel Ojo is the Co-Founder and Managing Director at SHEFFA Limited. He is presently studying Computer Science at the National Open University, Victoria Island, Lagos.