Question

I just received a 'positive' report of my skin test for tuberculosis. I have scheduled myself for a chest x-ray next week. Can the skin tests ever be a 'false' positive and what can I expect to result from this?

Answer

The tuberculosis skin test is a test to see if someone has ever been exposed to tuberculosis at any time in the past. The reason this is important is that when people inhale tuberculosis organisms only about 5% will become clinically ill within the first year. The other 95% either kill off the bacteria or it is harbored in small amounts in the body. Those people will usually have a positive tuberculosis skin test even though they do not feel ill. However, over time about 5% of these people will develop an active infection in their lifetime. So, the idea is to treat them before they ever get sick.

Keep in mind that most people will never get ill from the infection, even without treatment. Also, as you suggest, the skin test is not 100% accurate. It may be negative even though someone has been exposed, especially if his immune system is run down. It can be positive even if someone has never been exposed, especially if he was vaccinated for TB with the BCG vaccine (this type is not given in the USA). The larger the skin reaction, the greater the likelihood that it is a true positive test.

When there is a positive reaction, the person will be questioned about symptoms of active tuberculosis and a chest X-ray will be done to see if there is evidence of active or past infection. With this information, a decision can be made whether to treat the patient with anti-tuberculosis medications (usually for 6 months) or not.

Links

• Tuberculin Testing Policy