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Mycobacterium tuberculosis


          A tuberculin skin test is a diagnostic tool for detecting if an individual has been infected with Mycobacterium tuberculosis.  In the United States, the test if most commonly performed using the Mantoux method.  During the procedure, a tiny amount of purified tuberculin protein derivative (PPD) is injected just beneath the skin using a needle (Figure 1).  If a person tests positive for tuberculosis, a red, raised bump (induration) will develop within 48 to 72 hours (Figure 2).  The injection site is then examined by a healthcare professional who decides the next course of action based on the results.  An induration that measures between one and five millimeters is considered a negative response, so the test is repeated a week later using a stronger dose of PPD.  Sometimes, a false negative will result if a person was recently infected with the bacteria, so a tuberculin skin test proves most accurate if performed at least two to ten weeks after initial exposure.  Indurations that range from five to nine millimeters are considered questionable, so the test is also repeated a second time in this instance.  An induration measuring 10 to 33 millimeters indicates that a person has been infected with M. tuberculosis.

Figure 1. 
A tuberculin skin test performed using the Mantoux method.  The healthcare professional is injecting a tiny amount of PPD just beneath the skin.


Centers for Disease Control and Prevention. http://www.phppo.cdc.gov/phtn/tbmodules/modules1-5/m3/photos/3photo3.1.htm

Centers for Disease Control and Prevention. http://www.phppo.cdc.gov/phtn/tbmodules/modules1-5/m3/photos/3photo3.2.htm


Figure 2.
  Photograph depicting the red, raised bump that indicates a positive response to PPD.  The healthcare professional is measuring the induration to determine the degree of reaction.

         A positive skin test does not necessarily mean that an individual has tuberculosis disease, but rather that an individual has the bacteria inside of his or her body.  Therefore, people with latent tuberculosis infection and those who have had tuberculosis disease but received treatment will still test positive.  If a positive skin test is obtained, the healthcare professional will suggest that a chest x-ray be taken to look for signs of active tuberculosis disease (Figure 3).  Other confirmatory tests include staining and examining sputum samples or cultivating M. tuberculosis on laboratory media (Figure 4).  Usually, the culture method is utilized only in large hospitals or state public health facilities because the slow-growing bacterial colonies can take weeks to appear.

Centers for Disease Control and Prevention. http://phil.cdc.gov/phil/quicksearch.asp

Figure 3.  Chest x-ray of a patient with advanced pulmonary tuberculosis disease.  Calcification of the tubercles allows for visualization through radiographic imaging.  The patient exhibits the formation of a cavity in the right side of his lungs.  Such cavities result when the interiors of the tubercles become liquefied and spill out their contents.
Centers for Disease Control and Prevention. http://phil.cdc.gov/phil/quicksearch.asp

Figure 4.
  Colonies of Mycobacterium tuberculosis growing on laboratory media.