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The Tuberculosis Skin Test

Understanding the PPD


Updated May 27, 2014

Tuberculosis (TB) is a disease that is spread from person-to-person through the air, and it is particularly dangerous for people infected with HIV. Tuberculosis and HIV can be a deadly combination. In fact, worldwide TB is the leading cause of death among people infected with HIV.

Because of this fact, it is important for people living with HIV to get a TB skin test -- a routine part of HIV care. Let's take a closer look at tuberculosis testing.

TB Statistics

How is Tuberculosis Tested For?

The best known way to diagnose TB infection is to use the purified protein derivative standard (PPD test). A solution containing certain components of the bacteria that cause TB is injected under the top layer of skin. If a person is infected with TB, the injection will produce a delayed skin reaction to the solution.

Infection with TB causes specialized immune system cells, known as T-cells, to recognize the threatening bacteria. So, when the cells are exposed to the PPD, they respond by causing a skin reaction at the site of the injection.

How Exactly is the TB Test Administered?

A syringe with a very small gauge needle is filled with 0.1cc of the PPD solution. The solution is then injected between the layers of skin on the forearm. Unlike a vaccination that is given deep into the muscle, the PPD injection is so superficial that it leaves a raised area of skin (a wheal) at the injection site. If the injection doesn't create a wheal, the injection must be repeated. The wheal will eventually vanish over the course of several hours.

The injection must be made at an area of the forearm that is free of lesions, wounds, or veins.

Reading the PPD Skin Test

The skin reaction that results when a TB-positive person is injected is very specific to TB infection. Therefore, it can be used as a reliable way to identify people who have been infected with TB. The reaction reaches its peak about 48 to 72 hours after the PPD is injected -- the key time to examine the area. Waiting too long or not long enough may cause an underestimation of the reaction and, therefore, affect how the test is read.

A positive test will cause the following symptoms at the injection site:

  • Redness
  • Itching
  • Thickening of the skin (known as induration)
Presence or absence of induration, as well as the size of the induration, are the key indicators that set a positive result apart from a negative one. The extent of redness or bruising is not considered when determining the results of a TB test. Blisters or fluid filled lesions are also considered signs of a positive test.

Special Considerations When Reading a TB Test

Induration sizes that indicate a positive test result can vary under certain circumstances:
  • Healthy Immune System -- 15mm

  • Weakened Immune System - 5mm

  • Kidney Disease or Diabetes Sufferers/Healthcare Workers - 10mm

Understand the Immune Response

What Does it Mean if I Test Positive?

A positive test means that, at some point in your lifetime, you have been exposed and infected with TB. If you have a positive TB test, talk with your doctor about the next step. Your TB infection is most likely latent, meaning it is not active, contagious, or making you sick. Your doctor will most likely prescribe a course of antibiotics to manage your TB. In some cases, a positive test may indicate an active infection. If this is the case, your doctor will discuss symptoms, order any other necessary tests (such as a chest x-ray), and set a course of treatment.

Your Complete Guide to TB


Shiel, Wm. "Tuberculosis Skin Test (PPD Skin Test)"; MedicineNet.com; 24 Jul 2007.

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