Blood-tests Explained


Disclaimer:

This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

TB Screening Tests


At a Glance

Why Get Tested?

To screen for tuberculosis (TB)

When to Get Tested?

When you have a high risk of exposure to TB; sometimes as part of an examination prior to starting school or a new job (such as a college student, teacher, or daycare employee); when you have signs and symptoms consistent with TB

Sample Required?

For a tuberculin skin test, no sample is required. A small amount of purified protein derivative (PPD) solution is injected just under the first layer of skin of your inner forearm. For an interferon gamma release assay, a blood sample is drawn by needle from a vein in your arm.

Test Preparation Needed?

None

The Test Sample

What is being tested?

Tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium tuberculosis. TB primarily targets the lungs but may affect any area of the body such as the urinary tract, central nervous system, bones, joints, and/or other organs. TB screening tests help to determine whether a person has become infected with Mycobacterium tuberculosis bacteria. The screening tests measure the body's immune response to antigens derived from these 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 may cause an inactive (latent) infection or an active, progressive disease. The immune system of about 90% of people who become infected with TB manages 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 is not infectious but does have a "latent TB infection."

If, after some time, the immune system of an individual with an inactive infection becomes weakened (compromised), the mycobacteria may begin to grow again, leading to an active case of tuberculosis disease. Active TB does cause illness and can be passed to others through respiratory secretions such as sputum or aerosols released by coughing, sneezing, laughing, talking, singing, or breathing.

Both the tuberculin skin test and the IGRA blood test can detect M. tuberculosis infections, but neither can distinguish between latent and active infections. Additional tests, such as AFB testing, are required to help establish a diagnosis of an active TB infection.

How is the sample collected for testing?

For the tuberculin skin test, no sample is required. The test is performed on a person's skin. A purified protein derivative (PPD) solution that contains M. tuberculosis antigens, but not live bacteria, is used to provoke a hypersensitivity skin reaction (a red, raised bump) in those who have been infected by TB.

A health practitioner will wipe the inner forearm with alcohol and let the skin dry. Using a 1cc syringe and a tiny needle, the health practitioner will then inject a small amount of PPD solution just under the first layer of the skin. When done correctly, the injection forms a small bubble of fluid that looks like a blister. The site should be left uncovered and undisturbed. The site must be examined by a health practitioner at 48 and/or 72 hours to see if a local skin reaction has occurred.

For an interferon gamma release assay, a blood sample is obtained by inserting a needle into a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed. 

The Test