if , a person who was earlier affected by tuberculosis , is being tested in tuberculin test .then why does the person have swelling and redness at the site of injection after some time ???also why this test is not useful to find the presence of active tuberculosis ??

Dear student

Please refer the solution below for the said query

Tuberculin is a sterile protein extract of tubercle bacterium. It is administered in skin to test if someone has developed an immunity to bacterium causing TB.

There may be many reasons for the swelling and redness at the site of injection of a person who was earlier affected by tuberculosis. Some of them are:

1. Presence of latent infection i.e presence of some infection still in the patent.
2. If a person has received a  BCG vaccine may have swelling and redness
3. If a person has immunosuppression etc.

It is not useful to find active tuberculosis because it may give false positive results with conditions mentioned above. Also the reaction to tuberculin test is suppressed by various agents like live virus vaccine, malnutrition, ​ immuno-suppressive treatment  etc.



Hope this information clears your doubt about the topic.

Regards