How often should I test/retest my home for radon?

Our general guidance (A Citizen's Guide to Radon - (December 2016, EPA 402/K12/002) - suggests:

If your living patterns change and you begin occupying a lower level of your home (such as a basement) you should retest your home on that level.

Even if your test result is below 4 pCi/L, you may want to test again sometime in the future.

If you are buying or selling a home (from our Home Buyer's and Seller's Guide to Radon - (March 15, 2018, EPA 402/K-13/002):

If you are thinking of selling your home and you have already tested your home for radon, review the Radon Testing Checklist to make sure that the test was done correctly. If so, provide your test results to the buyer.

No matter what kind of test you took, a potential buyer may ask for a new test especially if:

The Radon Testing Checklist items were not met;
The last test is not recent, e.g., within two years;
You have renovated or altered your home since you tested; or
The buyer plans to live in a lower level of the house than was tested, such as a basement suitable for occupancy but not currently lived in.

A buyer may also ask for a new test if your state or local government requires disclosure of radon information to buyers.