Tuberculosis (TB) is an infectious disease which causes the death of around three million people every year. When certain strains of Mycobacteria infect the lungs, it can progress into this active disease and cause death when left untreated. Symptoms of tuberculosis include chronic cough with blood-tinged sputum, fever, night sweats and weight loss.

A person with these symptoms must immediately seek medical attention to confirm whether they have TB or not. There a number of ways in which tuberculosis is detected. A doctor can require patients to undergo chest X-rays, blood tests, as well as microscopic examination of bodily fluids. Another way is the tuberculin skin test (also called tuberculosis or PPD skin test). The bacterium that causes TB, M. tuberculosis, triggers an immune response which the tuberculin skin test detects. A response could mean that the person currently has TB or was previously exposed to it. After exposure to the TB bacteria, it takes a period of one to three months before the tuberculin skin test can yield valid results. 

How It Is Administered

A small amount of TB protein (antigens) is injected immediately under the surface of the skin, on the inner forearm. This TB antigen is called purified protein derivative (PPD), a dried form of tuberculin which may produce a tuberculin reaction (wheal) within 48-72 hours. 

Administering the test

Administering the test
Credit: Wikimedia

Reading the Reaction

Since this is a skin test, the result would warrant a change in the appearance of the skin where the PPD has been injected. Reading the skin test means detecting an induration – a thickened, raised area of skin. Redness and bruising is a completely different reaction. The induration is at its maximum 48-72 hours after the test, thus reading should be done within this time period.

The size of the induration is one factor that qualifies a person for tuberculosis treatment. Measuring the induration involves several techniques, including a ballpoint pen technique. Using a ballpoint pen, a line is drawn one to two centimetres from one end to the center of the induration. The pen is lifted when an increased resistance to further movement is felt. The same procedure is done on the opposite side. The distance between the ends of the two lines is the measurement of the induration.

Interpreting the Results

Positive Skin Test Reading

A positive tuberculin skin test result is determined by two factors: the size of the induration and the presence of risk factors. A doctor would recommend tuberculosis treatment if the TB skin reading as well as personal risk factors yield complementary results.

The first risk group includes people who are immunosuppressed, HIV positive, had recent contacts who are TB positive, with fibrotic changes on a chest radiograph consistent with old TB, and received organ transplants. They are given treatment for TB when their reading is greater than 5 mm of induration.

The second risk group includes injection drug users, residents and employees of high-risk congregate settings, mycobacteriology laboratory personnel, diabetics or patients of clinical conditions, children under four years or exposed to adults in high-risk categories and immigrants in high-prevalence countries. Treatment for TB is only recommended when the induration is greater than 10mm.

A person with no known risk factors with a positive tuberculin skin test result reading of 15 mm may also be recommended for latent tuberculosis treatment.

Negative Skin Test Reading

People with no known risk factors with an induration of less than 5 mm yield a negative tuberculin skin test result. However, there are some cases when a negative skin test reading becomes false negative. A person with cutaneous anergy or the inability to react to skin tests because of weak immune system, TB exposure less than eight to twelve weeks, or recent live-virus vaccination will most likely have a false negative tuberculin skin test result. Other reasons include very old TB infection, viral illnesses, very young age or infancy, and incorrect method of tuberculin skin test.

Positive test result

Positive tuberculin test result
Credit: National Institute of Health (NIH)

Negative test result

Negative tuberculin skin test result
Credit: Wikimedia Commons