Question

You have just been accepted to nursing school. As a requirement, you have to have a tuberculosis (TB) test as well as a hepatitis B recombinant vaccine. The nurse who administers the TB skin test explains that if significant swelling occurs around the injection site, you will need to have chest x-rays to determine if you are infected with Mycobacterium tuberculosis. On the morning of the second day after the skin test, you awake to find your arm red and swollen in an area about the size of a quarter around the site of the test. It is also tender to the touch. Now you are really worried. Could you have TB?

1. Why does the reaction to the skin test take 36-48 hours to show up? Explain. 2. If you have a tuberculosis infection, why doesn’t the whole body, or at least the respiratory tract, react when the antigen is injected during this diagnostic test?

Answer 1

• There is a chance of TB positive in you as the TB skin test showed a positive reaction.

1. The TB skin test also known as Mantoux test usually take 36 to 48 hours to show up the result because this purified protein derivative (PPD) exploits or destroys the delayed hypersensitivity reaction in a patient previously or already affected with mycobacteria.

• A positive reaction occurs that is redness ( induration) due to the presence of mononuclear cells at the site of injection which it takes to appear in the skin about 24 to 72 hours after injecting the PPD.
• Also some lymphokines gets attracted with this mononuclear cells and due to this reason TB reaction shows a characteristic erythema in the skin.

2. It is mainly because of you may be having a latent TB infection due to which your body or the respiratory tract are not reacting with the antigens injected.

• In latent TB infection, you will not feel sick or have any symptoms in your body even though you have TB bacteria in your body.
• The only positive reaction will be the test positive for TB skin test which makes the diagnosis of TB.
• Also the chest X ray and sputum test will be negative during latent infection.
• This is the reason why your body doesn't react to the antigen test even though you have TB bacteria in your body.

Note : TB or tuberculosis is a communicable disease spread through mycobacterium tuberculosis bacteria which is ruled out easily through a mantoux test or TB skin.