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What causes tuberculosis?

Q: My 65 years old grandmother was recently diagnosed with tuberculosis. She underwent PPD test or Mantoux test, which showed induration measured 8 mm. Chest x-ray is normal. What causes tuberculosis? Can it be treated completely?

A:Mantoux test using the Purified Protein Derivate (PPD) is a type of tuberculin testing which measure not the immunity but the delayed hypersensitivity to tuberculin. The test involves the intra-dermal injection of an antigen (tuberculin PPD) into the forearm and is read 48-72 hours later for the presence or absence of induration and its diameter. A Mantoux reaction between 6-15 mm can be due to previous exposure to the bacteria causing TB (Mycobacterium tuberculosis), exposure to non-tuberculous environmental mycobacteria, or due to previous BCG vaccination. A reaction greater than 15 mm is unlikely to be due to exposure to environmental non-tuberculous mycobacteria or due to BCG vaccination, and is more likely due to exposure to TB.

There are certain conditions which can give rise to false negative Mantoux tests (e.g. some viral infections such as measles, HIV infection, chemotherapy, disseminated TB, new born infants, corticosteroid treatment, chronic renal failure, diabetes, general debility, improper testing procedure etc). If you are generally well and healthy, and do not have any signs or symptoms (e.g. fever, cough, weight loss etc.) then there is no need for concern. In case you develop any of these symptoms or become unwell then you should consult a doctor. With regard to your grandmother, a patient undergoing effective anti-TB treatment becomes non-infectious usually after 2 weeks.

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