To stay on the good side of the EPA guidelines, any person or organization must calibrate the radon detector - or if using a passive system, provide calibrate spikes to determine accuracy, at least annually to ensure the testing equipment is giving valid results. What this means at a practical level is that for three weeks of every year, my radon detector is not available. I use a pair of Sun Nuclear 1028 model monitors to provide testing in Moscow (an EPA Zone 1 area) and Pullman (an EPA Zone 2 area). To calibrate them, I have to contact Sun Nuclear, arrange for the service and return shipping (about $160 per unit), package it up on my end to send to Florida and then sit, twiddling my thumbs while they calibrate the radon detector.
That's for one unit - I get to repeat this for the second unit as soon as the first gets back. I deliberately run a staggered cycle so I always have one unit ready to go.
Sun Nuclear also sends back a Radon Certificate of Calibration for each unit. If you want to read an example, here is the Radon Certification of Calibration for one of my units. Obviously, this one will be sent out shortly for fine-tuning as the cert is up on November 2nd.
One significant point is that there is no tracking of calibration done in either Washington or Idaho. This means that if you are a buyer or a seller (why sellers? see below!), you need to verify that the machines in use are properly calibrated so that you get accurate results. Ask to see the paperwork - don't take the inspector's word for it. As much as I want to believe every inspector is from Lake Wobegon and is above average, the reality is that there are some folks who refuse to follow this simple process.
It is crucial for you to verify the accuracy of the equipment used in your home. Should the results come back at a level that the EPA recommend remediation, you might lose the sale with the reputation of your home damaged (worst case) or be on the hook for the cost of the remediation - $1,800-$3,000 in my area.
Ask to see the documentation as part of the radon report - my particular equipment gets the date reset by the testing lab when they calibrate the radon detector. This isn't true of all labs so ask, ask, ask for the paperwork.