The reason for two step PPD testing is that if you are exposed, the reaction to the test gets worse each time. You might have no reaction, or an iffy reaction, the first time around. So to be sure, you get it twice. Our patients get the same treatment when they are admitted.
It's like a reminder to your immune system that, yes, indeed, you WERE exposed somewhere in the distant past, have antibodies, and can prove it by having a reaction to your PPD.
It is useful if you are exposed to active TB [at work] in the future, the hospital knows that you didn't convert from that exposure, so they are not liable. It is also useful if you decide to do INH prophylaxis to protect your own health.
CDC has been recommending a two-step TB skin test since 1994. The reason is that a false negative could be obtained on the first run, but stats show that a second test should be run to confirm the first test to be sure. Check out the CDC's website to double check, if you'd like.
I thought the same thing when our admissions policy changed when we got a new Dean. I thought she was crazy until I went to the CDC site and confirmed that information.
You do INH prophylaxis, if you converted from negative to positive PPD, and you have negative CXR, and no TB sx. Or if you're HIV-positive, but there are many debates about this. INH prophylaxis is given to help prevent you from getting active TB. Except in those over 35yrs of age, and have a normal CXR.
Some people turn down INH prophylaxis, d/t the liver damage that INH can cause. But it has been argued that proper and close monitoring of LFTs makes it safe to take INH, if you don't have any liver damage already.