Tuberculin Skin Test: Introduction, Procedure, Normal Range and Result

Tuberculin Skin Test: Introduction, Procedure, Normal Range and Result Interpretation

Introduction of Tuberculin Skin Test

Tuberculin skin test (TST) is also familiar with tuberculin test or tuberculosis skin test or Mendel–Mantoux test or tuberculin sensitivity test or purified protein derivative (PPD) tests. It is a tool for screening for tuberculosis and for tuberculosis diagnosis. The standard recommended tuberculin test is the Mantoux test and it determines if someone has developed an immune response to the bacterium that causes tuberculosis i.e. Mycobacterium tuberculosis. PPD uses in this test is the extract the components of the organism from TB cultures. This response can occur if someone currently has tuberculosis, if they were exposed to it in the past, or if they received the BCG vaccine against tuberculosis. Estimates indicate that one-third of the world’s population has latent tuberculosis (TB), and around 1.3 million people worldwide die of TB each year.

Requirements for Tuberculin Skin Test

Tuberculin vial (0.1 mL of a liquid containing 5 TU)
Needle and syringe ( a 27-gauge needle and a tuberculin syringe)
Marker
Alcohol swab
Test patient

Procedure of Mantoux test

Select the site.
Clear injecting site with alcohol swab from center to periphery.
Inject 0.1 mL of a liquid containing 5 tuberculin units of PPD into the top layers of skin of the forearm.
After that, you will observe discrete, pale elevation of the skin 6-10 mm in diameter that denotes good procedure and is generally quickly absorbed.
Mark the site with a marker and advice the patient not to touch and apply soap water or anything on that area.
Say the patient to follow up after 48-72 hours.

Observation of Tuberculin Skin Test

Read Mantoux test 48-72 hours after the injection for the presence or absence and the amount of induration i.e. localized swelling). Measure the diameter of the induration perpendicular to the long axis of the forearm and recorded in millimeters (mm) and ignore to measure only redness area.

Normal range of Mantoux test

Induration: < 5 mm

Result interpretation of Tuberculin Skin Test

Induration: < 5 mm is negative

Induration: ≥ 5 mm is positive in following cases like immunosuppressed individuals like HIV-infected person. People with changes seen on chest X-rays that are consistent with previous tuberculosis. Recent contacts of people with tuberculosis and people who are in organ transplants.

Induration: 10 mm is positive in recent immigrants from high-prevalence areas
residents and employees of high-risk areas , IV drug abusers, children under 4 years old, people working in TB laboratories

Induration: ≥15 mm is to be positive in a healthy person whose immune system is normal.

Keynotes on Tuberculin Skin Test

The skin test is the preferred test in children under 5 years of age.
The tuberculin skin test is based on the fact that infection with M. tuberculosis bacterium produces a delayed-type hypersensitivity skin reaction to certain components of the bacterium.
Reaction in the skin to tuberculin PPD begins when specialized immune cells, called T cells, sensitized by prior infection, are attracted by the immune system to the skin site where they release chemical messengers called lymphokines. These lymphokines induce induration through local vasodilation leading to fluid deposition known as edema, fibrin deposition, and attraction of other types of inflammatory cells to the area.
An incubation period of two to 12 weeks is usually necessary after exposure to the TB bacteria in order for the PPD test to be positive.
Anyone can have a TB test, and physicians can perform the test on infants, pregnant women, or HIV-infected people with no danger. It is only contraindicated in people who have had a severe reaction to a previous tuberculin skin test.
If it becomes apparent that the first test was improperly administered, another test can be given at once, selecting a site several centimeters away from the original injection.
On the other hand, a negative test does not always mean that a person is free of tuberculosis. People who have been infected with TB may not have a positive skin test (known as a false negative result) if their immune function is compromised by chronic medical conditions, cancer chemotherapy, or AIDS.
A person who received a BCG vaccine (administered in some countries but not the U.S.) against tuberculosis may also have a positive skin reaction to the TB test, although this is not always the case. This is an example of a false-positive result.
The positive reaction that is due to the vaccine may persist for years. Those who were vaccinated after the first year of life or who had more than one dose of the vaccine have the greatest likelihood of having a persistent positive result than those who were vaccinated as infants.
People infected with other types of mycobacteria other than Mycobacterium tuberculosis may also have false-positive TB skin tests.
The test typically does not produce side effects. Allergic reactions are also rare complications. Since the test does not use live bacteria, so there is no chance of developing tuberculosis from the test.

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