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Friday, July 3, 2015
Negative skin test after preventivetreatment
I had a positive tb skin test (four prong) as a 2 year-old, with no obvious risk of exposure. My parents treated me with the full-year course of whatever the treatment at that time -- 1970 -- was. My skin tests continued to be positive for several years after this and I remembering receiving at least two chest x-rays during my youth -- all fine. I was told that the skin tests would remain positive all my life. As a late teen however they became negative -- no longer the old 4-prong test -- and have remained negative ever since -- I am now 38.
No health worker/doctor has adequately explained this to me. Should I assume that the original test was inaccurate and that I had no exposure to tb as a child? Was I exposed but successfully treated and my immune response to tb eventually waned? Should I be getting chest Xrays because in fact the skin test is inaccurate after treatment and I am still at risk of infection?
There are 2 possible explanations, both of which you referred to.
The 4 prong test or Tine test was not a very good test. So it is possible that the positive Tine when you were a child was wrong and you were never exposed. The other possibility is that your treatment occurred soon after exposure and that the skin test reverted to negative. This can occur in up to 1/3 of people who are treated with isoniazid soon after exposure, especially children. However, if you test people who revert to negative over some period of time some of them will become positive again.
You write that your skin test has been negative since you were a teenager. As long as you do not have an immunosuppression disease (HIV infection, organ transplantation, take steroids or other immunosuppressive condition), you are probably not infected with tuberculosis. This means that you could be exposed again. So if you require testing for your job you could continue to get TB skin tests done. You do not need routine chest x-rays unless you have symptoms of active tuberculosis.
There is a new blood test now available in parts of the US called Quantiferon Gold. This tests for exposure to tuberculosis in another way. You could also consider this test to confirm the accuracy of your negative skin tests.
Catherine A Curley, MD, MS
Assistant Professor of Medicine
School of Medicine
Case Western Reserve University