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TB Skin Test


A member of our family had a positive reaction to a TB skin test, called the Mantoux, I believe. We are a little freaked out to say the least. What does a positive TB skin test mean and how does it impact our family? How worried do we need to be and what is our course of action?


Tuberculosis (TB) is peculiar. There is a distinct difference between being infected with the TB bacteria (latent TB) and having the disease called tuberculosis. From the moment of exposure, the organism ‘incubates’ in the body for about eight weeks. If the body contains the infection it remains dormant and silent (latent TB). Disease becomes apparent when the organism continues to grow in body tissues, most commonly the lung.

If 100 people are infected, only 10 will get the disease. The majority of people who develop the disease will do so within two years of being infected. However, TB can emerge at any time, influenced by nutrition, by medical conditions that weaken the immune system, or simply because of the impact of aging on the immune system.

Latent TB is diagnosed when the tuberculin skin test (TST or Mantoux) is positive but there is no evidence of clinical TB and a chest x-ray is normal. An important factor to remember is that a person with latent TB cannot spread the disease to others. This alleviates any concern for transmission of the latent form of TB to other members of the family who did not have positive TB skin tests.

Treating Latent TB

To treat or not to treat – that is the question. Several factors influence the decision to accept treatment for latent TB:

  • Under most circumstances, there should be a strong reaction to the TST, marked by a firm swelling at least 10 mm in size (a 2 cm of hardness is considered a strong reaction).

  • If there is recent conversion (going from negative to positive within a two-year period), one should seriously consider treatment since statistics show that most persons with the disease developed symptoms within two years of their infection.

  • Expatriates who plan to work in countries with high rates of TB may not benefit as much as they might think because treatment does not protect one from re-infection.

Treatment Concerns

There is a downside to treatment. Hepatitis (liver inflammation) will affect two per cent of people treated with the TB medication, especially those greater than 35 years of age. Therefore, persons with a history of liver disease and abnormal liver function tests should not be treated, and persons older than 35 should weigh the pros and cons more carefully.

Treatment Protocol

If a person chooses treatment, a liver test is usually done prior to the administration of the medication. If the test results are normal, the drug is administered daily for nine months. I usually test the liver at the end of one month, three months, and six months. Some consultants do not believe that is necessary. All patients are instructed to watch for the following signs of drug-induced hepatitis:

  • jaundice

  • abdominal pain

  • nausea

  • vomiting

If someone experiences any of these symptoms, he/she should contact their doctor immediately to discuss necessary actions or the possibility of terminating the treatment. If the person is remotely located, and no doctor is readily available, he/she should discontinue treatment. Some patients experience fatigue while on the treatment. This symptom is not dangerous, but at times, the fatigue is bad enough to make people want to stop taking the drug.

A Final Comment

In the end, treatment reduces the risk of disease by 80%. As in all areas of medicine, the treatment should never be worse than the disease.

Further reading

  • UpToDate on tuberculosis:

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