TB test question

Hi everyone,

I recently completed a two-step Mantoux test for a hospital volunteer position. I had no reaction either time. Ten years ago I worked as a CNA and during the initial TB screening I had what was interpreted as a big,fat positive but a negative CXR. My sister, who is a nurse, said I probably reacted to thermerisol or some other additive in the dose and not the tuberculin itself. Is this common? Has the test changed that much in ten years? I'm more curious than anything because for a long time I was convinced I was a positive reactor and at one point isoniazid as prophylaxis was suggested by health professionals. The neg CXR delayed that, but I would like to know why at one point I had an apparant positive and now no significant rxn at all.

Thank you to anyone who can help me out with this. :)

This is quite common, so people react at one point and not at others, I have heard of this happening before, your sister is probably right, sometimes as time passes we become more immune to things and thus don't respond in the same manner, just thank God you don't have the thing....