Radon is a gas that comes up from under ground. The house needs to be tested...the EPA standard is less that 4.0. If it is higher levels than 4.0 you can have the home mitigation system installed. If the system's fan is running is this removing all of the radon gas to the outside. You don't need to test again as long as the fan keeps running. (also if the gauge is at different levels you know it is working).
You can read up on radon on the EPA website.... more