The PPD is a skin test to test for tuberculosis. A 2 step process is required, initially, then a one step yearly. You get the PPD skin test (1st step), then 2 weeks later, you get the 2nd one (2nd step). Sometimes, if for instance you change jobs, and cannot provide them with proof of prior PPds, then they will have you take a 2 step.
this is a federal healthcare guideline.... it helps to protect the people that you (any healthcare worker) come in contact with. As fo why they require a 2 step, I believe that it is incase you have come in contact with a case of TB in recent days.
I think we'll see more places requiring two-step PPD as time goes on, esp in nursing homes and other high-risk areas.
The two-step gives a more accurate baseline and helps to reduce the incidence of false negatives. It helps "remind" the immune system of previous exposures and helps tell if it's old or new TB exposure. After the two-step, then just a once a year check usually, or maybe a check post known exposure, depends on your facility