What Causes a False Positive TB Skin Test?

TB (an abbreviation for Tubercle bacillus) is caused by Mycobacterium tuberculosis; a slow growing, wax-coated acid-fast bacterium that can remain viable in dried aerosol droplets for eight months. Although not considered highly infectious, it takes few M. tuberculosis bacteria to start a disease process that can destroy lung tissue, disseminate and result in death.

The Mantoux Tuburculin Skin Test

Those in the medical field, and others with a high risk of contracting tuberculosis, are routinely tested with the Mantoux tuberculin skin test (TST). The TST involves injection of a minute amount of tuberculin purified protein derivative (PPD) into the inner surface of the forearm.

How the TST Works

The immune system of those infected with the Mycobacterium tuburculosis will react to the PPD protein, which is an antigen derived from the bacteria's cell wall. For those with TB, the TST mimics reexposure to the pathogen, and reveals the rapid immune response of antibodies that would be present in a person who had previously been exposed to TB.

The test is read 48 to 72 hours after administered. Interpretation should only be done by a trained medical professional and involves evaluation of the injection site for a hard, red swelling indicative of a positive reaction.

False Positive TB Test

A positive TB test does not necessarily mean that a person has tuberculosis. There are several factors that can produce a false-positive TB skin test (TST) reaction.

Infection with Nontuberculous Mycobacteria

Although there are other, less common species within the same genus that can also cause TB, there are many other species of Mycobacterium that do not cause tuberculosis at all. A person who has been infected with another member of this genus may falsely test positive, since the PDD antigen used in the TB injection is common to other nontuberculous Mycobacteria.

BCG Vaccination

Although not yet used in the US due to questionable effectiveness, BCG (Bacille Calmette Guerin) vaccination is a TB immunization. Past inoculation with this vaccine can result in a positive TB test.

Booster Phenomenon

The TB "booster phenomenon" occurs because the immune system of someone previously infected with TB may lose its ability to react to tuberculin over time. Years later, when that person is skin tested again, the new test will come up negative, but the act of testing reminds the immune system about the previous infection.

If TST tested again, within a year, that person may have a positive reaction, because the first skin test essentially jogged the memory or the immune system, in effect, boosting its ability to react to tuburculin. This phenomenon is mainly seen in previously TB infected older adults.

Bottom line, a positive tuberculin skin test is not sufficient to distinguish among those:

• with a previous or current Mycobacterial infection other than M. tuberculosis
• who have been immunized with BCG
• previously infected but with no current infection
• with a latent TB infection
• with an active TB infection

Furthermore, it is possible for an incorrectly administered test appear positive or for a properly administered test to be falsely interpreted as positive.

The Next Step after a Positive TST

Due to the real possibility of a false-positive Mantoux tuberculin skin test, a positive result may be followed up with a chest x-ray or CT scan to reveal if there is an active infection present through evidence of lung cavities, nodules and fibrotic scars. A sputum test can also be done to determine if there is active infection, indicated by the presence of pathogenic Mycobacterium growing in strands, called cords.

Sources

CDC Tuberculosis Website (2008) Division of Tuberculosis Elimination.

Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.

* This article, originally published in Suite101 online magazine, is not meant to be used in the diagnosis or treatment of illness. If you believe that you are sick, see s doctor, not a computer.