Think about how a medical lab works. A doctor orders a test, they run that test as designed by the manufacturer, print out the results, fax them back to the doctor, and the doctor interprets them.

When laymen contact a lab with a special request, they generally are asking the lab to take on all the roles from test selection to interpretation. In our medical example that would mean the lab employing a qualified doctor, instead of just lab technicians and assuming the liability of the same.

Any tests re-inforce the delusion no matter what the results are.

If skin is sent off and tested and nothing at all is fou d the delusion will make people think things like "science just doesn't know the right test"; "the bugs live in a deeper layer of skin"; "the test wasn't looking for the right things"; "it's a conspiracy of the medical establishment to deny the existence of these parasites so they can push drugs".