Page 204 - fourth year book

COMMUNICABLE DISEASES (T.B)

False-positive and False-negative skin test reactions

False-positive False-negative

Definition Persons may react to the TST Persons may not react to the TST

even though they are not even though they are infected

infected with M. tuberculosis with M. tuberculosis. This is

called anergy: inability to react to

skin tests because of a weakened

immune system

Causes  Infection secondary to  Recent TB infection (within 8-

other species of TB 10 weeks of exposure)

bacteria  Very old TB infection (many

 Previous BCG vaccination years)

 Incorrect method of TST  Very young age (less than 6

administration months old)

 Incorrect interpretation of  Recent live-virus vaccination

reaction  Viral illnesses (as measles and

 Incorrect bottle of antigen chicken pox)

used  Incorrect method of TST

administration

 Incorrect interpretation of

reaction

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