Tuberculosis is an infectious disease that transmits through saliva droplets that come out when we either breathe, cough, or speak. Among all respiratory illnesses, tuberculosis is one with the highest mortality rate.

Besides affecting the lungs, tuberculosis can also affect other body areas like joints, bones, kidneys, or the brain (central nervous system).

In some cases, tuberculosis patients won’t present symptoms of illness, this is known as silent (latent) tuberculosis. Silent tuberculosis is not an infectious disease. However, an early diagnosis is crucial to the preservation of good health.

To diagnose latent tuberculosis, labs use a method called Purified Protein Derivative Testing, commonly known as PPD testing.

In this article, we will elaborate further on what is PPD testing: its procedure, results, and side effects.

How to prepare for PPD testing

Before taking the test, please make sure to talk with your healthcare provider about the next issues:

  • If you have tested positive for a PPD test, it is not recommended for you to take the test again. If this is the case, tell your healthcare provider, they may suggest a blood test instead of a PPD test.
  • Talk to your healthcare provider about any medical conditions you have. Also, tell them if you are taking either medications or supplements since these could affect your test results.


Before proceeding with the test, your doctor will do a checkup of your breathing quality (lungs) and if there is any swelling on your neck area (lymph nodes). If necessary, your doctor will order a PPD test to confirm the diagnosis.

Here is what you can expect during the test:

  1. A healthcare provider will inject a liquid known as tuberculin — which contains a purified protein derivative. The shot is usually done gently in the inside of the forearm, under the top layer of the skin.
  2. The injection will cause a small bump (welt) to appear. Once the tuberculin is absorbed, the bump will naturally disappear in a few hours.
  3. You must come back after the course of 2 or 3 days for a healthcare professional to check if there is a skin reaction where you got the injection.

Result Interpretation

There are four types of results you may expect from a PPD test:

  • Negative. When the skin doesn’t show any reaction to the PPD shot. This means you don’t have tuberculosis.
  • Positive. A positive PPD test will show signs of either swelling or hardness on the skin. If the swelling signs are present, a healthcare professional will measure the swelling area with a ruler.
  • Abnormal Positive. These results mean that you have been infected with TB bacteria in the past. However, tuberculosis is not currently active. More tests will be required to confirm the disease is not active. Treatment may be necessary to prevent the reactivation of tuberculosis.
  • False Negative. Unfortunately, the PPD test is not 100% accurate. In rare cases, positive TB infections don’t cause any reaction to the skin. A false negative may be the result of having diseases or taking medicines that are currently compromising your immune system.

Side Effects 

Besides swelling, people can experience severe redness on the arm where the shot was applied. This usually happens to people that have had a positive PPD test before. However, on rare occasions, this side effect could also happen to people who are taking the test for the first time. 

Soteria Screening Laboratories count with qualified healthcare providers to assist you in getting the necessary tests and assure an accurate diagnosis. Feel welcomed to reach out to us to clarify any doubts or concerns regarding TB tests. We are here to help!


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