A person recently infected with TB may not react to the TB skin test. This is also true of elderly, debilitated and immunocompromised (e.g., AIDS) patients. In the case of patients who fit into these categories and who are suspected to be infected with tuberculosis, other tests including a chest x ray or a skin test at a later date may be used to determine the presence or absence of TB infection and disease



Contacts who are skin tested less than 10 to 12 weeks after their last exposure to a patient with infectious TB may have a false-negative reaction, because they may not yet be able to react to the tuberculin. It takes 2 to 12 weeks after TB infection for the body's immune system to react to tuberculin. For this reason, close contacts of someone with infectious TB disease who have a negative initial skin test reaction should be retested 10 to 12 weeks after the last contact with the person who has TB disease. The time span between the date of an initial skin test with a negative reaction and the date that is 10 to 12 weeks after exposure is called the window period. After the window period has ended, a repeat skin test should be administered to each contact who had an initial negative reaction
Check this link for details
http://www.cdc.gov/nchstp/tb/pubs/ssmodules/module6/ss6study.htm


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Edited by dr_amitmunjal on June 25 2006 at 4:05 AM

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