DEAR DR. ROACH: I had a tuberculosis (TB) skin test for my new job in May, and it was negative. They did a blood test in July. I have a question about a positive T-SPOT blood test for TB. They did a chest X-ray, and it shows nothing wrong. Is there any treatment necessary? I haven’t been told of any, but from what I read, it could be latent TB, which means it’s just silent but can show up at any time. Does this mean that I came into contact with TB and my body just fought it off, or could this be a false positive? Any help would be great. — A.M.
ANSWER: The T-SPOT, like the QuantiFERON-TB Gold, is a blood test to look for tuberculosis infection. Tuberculosis is an infection known for millennia. “Active” tuberculosis and “latent” tuberculosis are different, and the blood tests will be positive in both active and latent TB. Active tuberculosis commonly causes cough, fatigue and weight loss (centuries ago, it was called “consumption”). A chest X-ray is abnormal in virtually all cases of active tuberculosis: Since yours is negative (and assuming you have no symptoms), it’s very likely you have latent tuberculosis. Both blood tests (these work by measuring the response of T cells to tuberculosis proteins) are more accurate than the TB skin test. Blood tests have the advantage of only coming in once, as opposed to the skin test where you need to have the test read by a clinician in 48 to 72 hours. The blood test is more than 95% specific, meaning it’s extremely likely that you have latent infection and extremely unlikely to be a false positive. One disadvantage is that the blood test is much more expensive than the skin test.
Latent infection means the tuberculosis bacteria are still alive inside you, but kept in check by your immune system. Unfortunately, due to illness, stress or just getting older, sometimes latent infections can escape the immune system and become active infections. Roughly 8% of people with latent TB will develop active TB during their lifetimes, but it’s most common within a few years of acquiring the latent infection.
People of any age who recently acquired latent TB (meaning their previous tests were negative, and now they’re positive, called “converters”) are at high enough risk for active TB that they are recommended to receive treatment. This is intended to kill all the latent TB bacteria and reduce the likelihood of developing active TB later. Treatment is highly effective, with most studies showing only about a 1% lifetime chance of developing active tuberculosis after completing treatment for latent TB. There are both one-drug and two-drug regimens for latent TB, with two-drug regimens lasting three months, and one-drug regimens lasting four to nine months. The most commonly used regimen now is probably rifamycin for four months, which is both very effective and minimally toxic.
People who have a positive test without a recent negative test (called “reactors”) are also usually treated, but there are some conditions where the risks and benefits of treatment need to be carefully weighed to make an individualized recommendation. But if someone ordered the test, they should also have made the decision that it’s worthwhile to treat you if the result is positive.
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Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to [email protected] or send mail to 628 Virginia Dr., Orlando, FL 32803.
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