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Friday, August 1, 2014
TB skin test (PPD)
What does a positive TB skin test (12mm) followed by a negative skin test (0mm)mean? No symptoms present.
The tuberculosis skin test is not a perfect test. Occasionally we can see a reaction to a test that is relatively small, like yours at 12 mm, that is negative (0mm) on repeat testing. There are a number of technical issues in the placement and interpretation of the test:
1. Which solution was used. There are 2 solutions available (aplisol and tubersol). Aplisol seems to cause more slight red reactions than tubersol.
2. Was the test done properly. Was a small bubble seen at the site of injection just after the test? This is called a wheal, and this is the correct method of placement.
3. Was the person who read the test experienced in skin test reading?
Assuming that both tests were done correctly and read correctly this is the way I would evaluate an individual with 1st test 12 mm, 2nd test 0 mm:
I would begin by deciding what the risk of TB exposure is. Were you born in the US or in a country where there is a lot of TB? (eg India or Africa). Are you a health care worker? Do you have any diseases that suppress the immune system (eg HIV infection).
If there is an elevated risk of TB exposure or a serious illness, then I would consider placing a 3rd skin test in a few months, or using the new Quantiferon Gold blood test to clarify the risk of TB infection.
If there is low risk of exposure to TB and no significant health issues then both the 12 mm reaction and the 0 mm reaction can be interpreted as negative for tuberculosis infection and no further evaluation is indicated.
Catherine A Curley, MD, MS
Assistant Professor of Medicine
School of Medicine
Case Western Reserve University