There are several things which can contribute to "faulty" radon test results. While there are a number of companies and individuals offering to perform "radon testing services" (home inspectors, termite and pest control technicians),  the EPA recommends that you only use companies that have individuals certified in EPA testing protocol to conduct your radon testing (especially involving real estate transactions).  All certified Radon professionals are required to follow EPA protocols to make sure that their work meets minimum quality/proficiency standards.  Certification in radon test measurement and in radon mitigation system design and installation requires contractors to take extensive class room training and pass examinations in each respective area before being listed as "proficient". There are also bi-annual continuing education requirements for program participants.  Due to a cut-back in funding by Congress in 1998, the EPA turned oversight of their proficiency (certification) program over to two independent organizations: NEHA, (National Environmental Health Association) and the NRSB, (National Radon Safety Board).  The contractor you use should have employees that are "listed" by one or both of these organizations as having passed proficiency examinations for radon test measurement (radon testing) and/or for radon mitigation (system installation).  A number of states also have their own contractor certification programs that have additional requirements.  Check with your state radon office to see if the contractor you are considering is state certified and/or NEHA or NRSB listed.  Remember, its the "individual" not the company that is "certified"  

Note:   Buyers Beware