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I had this test - but could not prove that I did not need it. I was immunized as a child more than thirty years ago in Yugoslavia and had no documentation.
So, a positive test indicates an induration - a raised red swelling. It is caused by TB antibodies. TB antibodies are present in those afflicted with the disease, or those who were immunized - the test cannot differentiate, obviously. If an induration is greater than 15 mm, then an x-ray is required. In my case, my x-ray was clean - as expected from an immunized individual.
For healthy people, I suggest that if you know you were immunized, to either produce the documentation to avoid the skin test, or else state the fact to your doctor, again to avoid the skin test, and go directly to the x-ray. Immunization papers or x-ray are required as proof - might as well skip the waste of money, time, and mild discomfort of the skin test.
By the way, the induration took five weeks to fade from my skin.
Good luck.