Tuberculosis screening tests | Pathology Tests Explained

What is being tested?

Tuberculosis (TB) screening tests help to determine whether a person has been infected with Mycobacterium tuberculosis bacteria, the cause of TB. The screening tests measure the body's immune response to antigens derived from the bacteria – either directly as a skin reaction to a tuberculin skin test (TST) or indirectly with an interferon gamma release assay (IGRA) blood test.

TB, once called "consumption", has been recognised as causing illness for thousands of years. This bacterial infection may affect many body organs, but it primarily targets the lungs. TB may cause an inactive (latent) infection or an active, progressive disease. The immune system of about 90% of the people who become infected with TB manage to control its growth and confine the TB infection to a few cells in the body. The bacteria in these cells are inactive but still alive. The person does not have any symptoms and they are not infectious, but they do have a "latent TB infection." If, after some time, the person's immune system becomes weakened (compromised), the mycobacteria may begin to grow again, leading to an active case of tuberculosis disease. Active TB does cause illness in the person and it may be passed to others through respiratory secretions such as sputum or aerosols released by coughing, sneezing, laughing, talking, singing or breathing.

The tuberculin skin test involves two steps: the injection of a small amount of purified protein derivative (PPD) solution under the first layer of skin of the forearm and an evaluation of the injection site conducted by a health care worker at 48 and/or 72 hours to see if a local skin reaction has occurred.

The IGRA test measures the release of gamma interferon by white blood cells in a sample of blood when the cells are exposed to specific TB antigens.

Both tests can detect M. tuberculosis infections, but neither can distinguish between latent and active infections.

How is it used?

TB screening tests are not used as a general population screens but are used to screen certain people who are at high risk for TB exposure, such as:

Those with diseases or conditions that weaken the immune system, such as those with HIV or AIDS , that make them more vulnerable to a TB infection
Those who have a condition for which treatment may weaken the immune system
Those who are in confined living conditions such as homeless shelters, detention centres, migrant hostels, nursing homes, schools, and correctional facilities
Health care workers and others whose occupations bring them in close contact with those who may have active TB
Those who have been in close contact with someone who has an active case of TB
Those who come from or have lived for a period of time in a foreign country where TB may be more common
Those who inject illegal drugs

TB screening tests are also used sometimes as part of a routine examination prior to starting school or a new job. Since mothers can pass TB to their unborn children, pregnant women are sometimes screened.

Testing is used to help diagnose a Mycobacterium tuberculosis infection, however it can not distinguish the difference between a latent TB infection or active disease. If the doctor suspects that a person has active tuberculosis, other tests are used to confirm the diagnosis, such as chest X-ray and Acid fast bacilli (AFB) smears and cultures.

Either a tuberculin skin test or an IGRA may be performed. Since the test requires viable white blood cells, the IGRA blood sample must be received and tested by the laboratory within a designated window of time.

When is it requested?

TB screening tests are ordered when a doctor wants to screen a person for exposure to TB. They may be done in people with a disease that weakens their immune system or because they work or live around others in high-risk groups. They are frequently done prior to a person starting an at-risk job, e.g. in a health care facility.

Since TB is airborne and passed through respiratory secretions when a person speaks, sings, coughs or sneezes, TB screening tests may be ordered when someone has been in close contact with someone who has an active case of TB or when they have been in a foreign country where TB is more common. This would be done a few weeks after suspected exposure as it can take up to 10 weeks after contact and initial infection before a positive result would be detected. TB skin tests should not be done when a person has had a previous positive reaction as they are more likely to have a severe local reaction.

Either a TB skin test (TST) or an IGRA may be ordered. The IGRA test should be performed when those tested are less likely to comply with returning to have their TST evaluated or when the person being tested has received BCG (Bacille Calmette-Guérin). BCG is a TB vaccine that can interfere with the interpretation of a TST. BCG is not now used routinely in Australia, but is administered in countries with a higher incidence of TB and it is used as a treatment for some types of bladder cancers.

TST is preferred when testing children under 5 years of age and both tests might be useful when someone with a negative initial TST or IGRA test has an increased risk for TB infection. Sometimes the TST is given in a two-step process. If there is a risk that the first TST is a false negative reaction, a second skin test is given so that the TST may stimulate the immune system, causing a positive or boosted reaction in the second test. This is typically performed for health care workers at the start of their employment.

What does the result mean?

A health care worker will interpret your tuberculin skin test results by looking at the injection site on your forearm at 48 or 72 hours (in most cases). A positive result will form a red and swollen circle at the site of the injection. The size (diameter) of the swollen raised circle determines whether exposure to TB has occurred. The size that is considered positive varies with the health status and age of the individual. Even when infected, children, the elderly, and people who are severely immune compromised (such as those with AIDS) may have smaller, delayed, or even negative reactions to the TST. Positive TST results are also commonly seen in those who have received a BCG vaccination.

IGRA test results may be positive, negative, or indeterminate. Positive results mean that a person is likely to have been exposed to TB. They may be due to a latent or active TB infection, or occasionally due to a false positive. IGRA results are not affected by BCG.

Negative results for either test may mean that a person has not been exposed to TB, that the person is not infected with tuberculosis, that their immune system has not responded to the antigen in the test, or that it is too early to detect exposure. It takes between 2 and 10 weeks after infection before a person demonstrates a positive reaction to TB screening tests. If a doctor wants to confirm a negative or indeterminate result, he/she may repeat the same test or do either the TST or IGRA as an alternate follow-up test.

Occasionally, a person infected with another species of Mycobacterium, for example Mycobacterium kansasii, will give a false positive IGRA result. Positive results must be followed up by other tests such as chest X-rays to look for signs of active TB disease. If active TB disease is suspected, AFB cultures from sputum may be used to confirm the diagnosis and determine the drug susceptibility for the M. tuberculosis infecting the patient.

Is there anything else I should know?

Once someone has had a positive TB skin test, it is not necessary to have another one the next time there is a question of exposure to TB. A TST reaction will usually remain positive, and the skin reaction to subsequent tuberculin skin tests may become increasingly severe.

A negative TST may cause mild itching or discomfort at the injection site. A person may not respond to a TB skin test (even with TB exposure) if she has had a recent viral infection, a "live" vaccine (such as measles, mumps, chickenpox, influenza), or has overwhelming tuberculosis, another bacterial infection, or is taking immune suppressive drugs such as corticosteroids.

Common questions

Does it matter whether I have a TST or a IGRA test?

In most cases, either test can be used. However, the results are not interchangeable so if you are at risk and your doctor is ordering TB screening on a periodic basis to monitor your exposure, then it may be preferable to do the same test each time for consistency and, if it is an IGRA, to send it to the same laboratory.

Should I get a tuberculosis screening test if I am pregnant?

Either TST or IGRAs can be performed safely during pregnancy however these tests are usually not necessary. Active TB, but not latent TB, can be passed from mother to child during pregnancy and so if there are concerns that you may have active TB, appropriate tests should be performed e.g. sputum TB culture.

What about the multiple puncture prong test for TB?

This is called the "tine" test and is rarely used any more. It involved the use of a device with multiple prongs/pins that were either dipped into a tuberculin solution and then pricked the skin, or pricked the skin through a drop of tuberculin that had been applied to the surface of the skin. The tine test was not considered as accurate because the amount being delivered could not be controlled. Any positive tine tests had to be followed up with the regular TB/PPD skin test.