Tuberculin skin test – PPD

What is being tested?
Tuberculosis (TB) is an infectious disease caused by a bacterial infection. It can affect many organs of the body, but mainly affects the lungs. Tuberculosis, caused by Mycobacterium tuberculosis and once called consumption, has been causing disease for thousands of years.
TB can cause an inactive (latent) infection or an active, progressive disease. In about 90% of people infected with TB, the immune system manages to control the growth of the bacteria and confine the TB infection to a few cells in the body.
The bacteria in these cells are inactive but still alive. The person has no symptoms and is not infectious, but has a “latent TB infection”. If the person’s immune status is compromised, the inactive mycobacteria may start to grow again, leading to active disease.
This active TB makes the person ill and can be passed on to others through respiratory secretions such as sputum or aerosols released when coughing, sneezing, laughing or breathing.
If the TB skin test is positive, the person may have active TB or a latent infection. Further tests will be carried out to diagnose active tuberculosis infection.

How is the sample collected for testing?
No sample is needed. The test is carried out on the patient’s skin. A purified protein derivative (PPD) solution containing Mycobacterium tuberculosis antigens, and no live bacteria, is used to trigger a skin hypersensitivity reaction (a red, swollen, hardened area) in people who may have been infected with TB.
A healthcare professional will clean the inner surface of the forearm with alcohol and let the skin dry. Using a 1 ml syringe and a small needle, they will inject a small amount of PPD solution just under the top layer of the skin. When done correctly, the injection forms a small bubble of fluid resembling a blister. The site should not be touched and should be left uncovered. The injection site must be examined by a healthcare professional 48 and/or 72 hours after the injection to see whether there has been a local skin reaction.

1. How is it used?
2. When is it requested?
3. What does the test result mean?
4. Is there anything else I should know?

1. How is it used?
The TB skin test is not used to screen the general population but to assess particular populations at high risk of TB exposure, such as: people with diseases or conditions that weaken their immune system, such as HIV or AIDS, making them more vulnerable to TB infection; people living in closed settings such as care homes, schools, etc.; healthcare workers and others whose jobs bring them into close contact with people with active TB infection; people who come from, or have lived for some time in, a foreign country where TB may be more common.
The TB skin test is also sometimes used as part of a routine check before starting school or a new job. The TB skin test can also be carried out on pregnant women, as they could pass TB to their babies during pregnancy. The TB skin test is used to diagnose latent TB infection or active disease. If the doctor suspects active tuberculosis, other tests, such as a chest X-ray and AFB cultures, are used to confirm the diagnosis.

2. When is it requested?
The doctor requests the TB skin test to check a patient after exposure to TB. The TB skin test may be done every year for people in high-risk groups, either because they have a disease that weakens their immune system or because they work or live with high-risk groups. TB skin tests are often done before a person joins a high-risk population, for example when starting university or becoming a teacher or healthcare worker.
Because TB is airborne and spread through respiratory secretions, the TB skin test may be requested when a person has been in close contact with a patient with active TB infection or has been in a foreign country where TB is more common. The test should be done a few weeks after the suspected exposure, as it takes about 6 weeks after contact and initial infection for the test to become positive.
The TB skin test should not be done when a person has had a previous positive reaction, as they are more likely to have a strong local reaction

3. What does the test result mean?
A healthcare professional will interpret the test results by looking at the injection site on the forearm at 48 or 72 hours (in most cases).
A positive result produces a red, swollen circle at the injection site. The size (diameter) of the circle determines whether there has been exposure to TB. The diameter considered positive varies according to the person’s state of health and age. Even when infected, children, older people and severely immunocompromised patients (such as people with AIDS) may have a small diameter, a delayed reaction or even a negative TB skin test.
Negative results may mean that the person has not been exposed to TB, that their immune system has not responded to the test or that it is too early to detect exposure. It takes about 6 weeks after infection for a person to have a positive PPD reaction. If the doctor wants to confirm a negative result, they may repeat the TB skin test or, in some cases, request another type of test called the QuantiFERON®-TB Gold test (QFT-G).
Positive results may be due to latent or active TB infection or, rarely, a false positive. Positive results may be seen in people who have had the BCG (bacillus Calmette-Guérin) vaccine.
BCG is a vaccine that is not used in the United States but is often given routinely in other countries with a higher incidence of tuberculosis.
Positive results must be followed by other tests, such as a chest X-ray, to look for signs of active TB disease. If active TB infection is suspected, AFB cultures can be used to confirm the diagnosis.

4. Is there anything else I should know?
Once you have tested positive on the TB skin test, you do not need another one if a new exposure to TB is suspected. Your PPD reaction will usually stay positive, and the skin reaction to later TB skin tests may become stronger and stronger.
The TB skin test may cause mild itching or discomfort at the injection site. A person may not respond to a TB skin test (even if they have TB) if they have had a recent viral infection or a “live” vaccine (such as measles, mumps, chickenpox and flu vaccines), or if they have acute tuberculosis or another bacterial infection, or are taking immunosuppressive drugs such as corticosteroids

1. I have heard of another test called the QuantiFERON-TB Gold test. Can it be used to diagnose latent TB infection?
2. Should I have the TB skin test if I am pregnant?
3. What is the multiple-puncture test for tuberculosis?

1. I have heard of another test called the QuantiFERON-TB Gold test. Can it be used to diagnose latent TB infection?
Yes, the FDA has approved a blood test called QFT-G (QuantiFERON®-TB Gold test). It can be used to help confirm or rule out latent or active tuberculosis infection. This test is not affected by previous QFT-G or TB skin tests or by BCG vaccination. The patient does not need to come back after 48-72 hours to have the result read by a healthcare professional, and there is no possible local skin reaction to a PPD injection, because there is no injection.
QFT-G requires a blood sample to be taken from the patient and processed (centrifuged and stored) in the laboratory within 16 hours of collection, which limits the test’s clinical usefulness and availability in many situations. There are still limited data on the usefulness of this test in some populations, such as children, people with a suppressed immune system and those recently exposed to TB.

2. Should I have the TB skin test if I am pregnant?
Only under medical supervision and only if it is needed. Not enough studies have been carried out to clearly document the safety of the PPD solution during pregnancy. However, because TB can be passed from mother to baby through the amniotic fluid during pregnancy, if the mother is at high risk of catching TB, the doctor may request a TB skin test.

3. What is the tine test for tuberculosis?
This test is rarely used today. It used a device with multiple prongs that were dipped in tuberculin solution and then pricked the skin, or pricked the skin through a drop of tuberculin previously applied to the skin surface. The tine test was considered inaccurate because the amount of tuberculin could not be controlled. A tine test had to be followed by a standard TB skin test