Adapted from Information from Centers for Disease Control and Prevention: http://www.cdc.gov/tb/topic/testing/default.htm

Tuberculosis (TB) is a disease caused by the organism Mycobacterium tuberculosis. There are two kinds of tests that are used to determine if a person has been infected with TB bacteria: the tuberculin skin test (TST) and tuberculin blood tests. A positive TST or TB blood test indicates that a person has been infected with TB bacteria. It does not tell whether the person has latent TB infection (LTBI) or has active TB. Physicians must follow up positive test results with chest radiographs, cytology of sputum, culture of sputum or lavage fluids, biopsy, etc.

Tuberculin Skin Test

The TST is performed by injecting 0.1 mL of tuberculin purified protein derivative (PPD) into the inner surface of the forearm by intradermal injection. A person given the tuberculin skin test must return within 48 to 72 hours to have a trained health care worker look for a reaction on the arm. The health care worker will look for a raised, hard area or swelling, and if present, measure its size using a ruler. Redness by itself is not considered part of the reaction.

The skin test result depends on the size of the raised, hard area or swelling. It also depends on certain risk factors of being infected and of progression to TB disease.

• Positive skin test: This means the person’s body is or has been infected with TB bacteria. Additional tests are needed to determine if the person has LTBI or TB disease. A health care worker will then provide treatment as needed
• Negative skin test: This means the person’s body did not react to the test, and that LTBI or TB disease is less likely

Some persons may not react to the TST even though they are infected with M. tuberculosis. The reasons for these false-negative reactions may include, but are not limited to, the following:

• Cutaneous anergy (anergy is the inability to react to skin tests because of a weakened immune system)
• Recent TB infection (within 8-10 weeks of exposure)
• Very old TB infection (many years)
• Very young age (less than 6 months old)
• Recent live-virus vaccination (e.g., measles and smallpox)
• Overwhelming TB disease
• Some viral illnesses (e.g., measles and chicken pox)
• Incorrect method of TST administration
• Incorrect interpretation of reaction

Tuberculin Blood Test

TB blood tests (also called interferon-gamma release assays or IGRAs) measure how certain circulating immune cells react to the bacteria that cause TB. An IGRA measures how strong a person’s immune system reacts to TB bacteria by testing the person’s blood in a laboratory.

Two IGRAs are approved by the U.S. Food and Drug Administration (FDA):

• QuantiFERON^®-TB Gold In-Tube test (QFT-GIT)
• T-SPOT^®.TB test (T-Spot)

A positive IGRA means that the person has been infected with TB bacteria. Physicians must follow up with additional tests to determine if the person has LTBI or active TB. A negative IGRA means that the person’s blood did not react to the test and that LTBI or TB disease is not likely. However, the IGRA may be falsely-negative in patients with immune suppression due to disease, conditions, or drugs.

IGRAs are the preferred method of TB infection testing for the following:

• People who have received a TB (BCG) vaccine (people from outside the U.S.)
• People who have a difficult time returning for a second appointment to assess for a reaction to the TST

Who Should Get Tested for TB

• People exposed to someone who has TB
• People with HIV infection or another medical problem that weakens the immune system
• People starting certain medications that suppress the ability to combat TB
• People who have symptoms of TB (fever, night sweats, cough, and weight loss)
• People from a country where TB is common (Central and South America, the Caribbean, Africa, Asia, Eastern Europe, and Russia)
• People who live or work around people at risk for TB in confined quarters (such as jails or certain health care facilities)
• People who use illegal drugs

¹Information from Centers for Disease Control and Prevention: http://www.cdc.gov/tb/topic/testing/default.htm.

Therapeutic Antibodies and Tuberculosis Testing

(The following is not from the CDC website)
Physicians have never before had such a powerful battery of approved and developing immune modulating therapies. Therapeutic antibodies represent some of the most important tools for managing autoimmune and inflammatory conditions. A certain number of these powerful therapeutics also have a profound effect on cellular immune function, vitally important to maintaining normal health and preventing disease such as TB, fungal infections, and parasitic infestations.

Drugs such as the TNF-α inhibitors (i.e., infliximab, adalimumab, certolizumab, golimumab, and etanercept) and other immune-modulating drugs have warnings about the need to test for TB prior to initiating therapy.

However, patients with diseases such as Crohn’s disease, psoriasis and psoriatic arthritis, rheumatoid arthritis, ankylosing spondylitis, and other conditions often have prior treatment with immunosuppressive drugs such as prednisone. These treated patients may not react properly to TSTs.

CANDIN^® can help assess a patient’s state of immune suppression or anergy, contributing to interpretation of tests such as TST. Because some persons with normal cellular immunity are not hypersensitive to Candida, concurrent use of other cell-mediated hypersensitivity skin test antigens is recommended.
See Full Prescribing Information.