I did a stool test with DiagnosTech, about 6 years ago, and it came back positive for 3 parasites.
The trick.... I went to the Lab in and gave them a fresh stool sample and asked if it could be tested within 20 minutes.
Dr Omar Amin, with parasite Testing in Scottsdale, has spoken about this issue in several lectures and mentions for most stool tests the sample needs to be tested within 20 minutes since the specific testing methods used by most American labs are not sensitive.
For some reason the FDA will not approve all the other more highly sensitive testing methods.