I agree with Roger, it would make a good conference session, anyone volunteers?
I think although the guidance for testing makes certain recommendations, it does allow you to relax the frequency based on historical trends. We've been slowly reducing our frequency based on the evidence built up over the years.
TVC/endo is done every six months at home and in-centre. After five years of being six monthly the trace metals mandatory suite has now gone to annual, done in July when the incoming water is at it's warmest.
On the very rare occasion we pick up an abnormal result (which has only ever happened at home locations, and only ever TVC/endo ) we've redone the test without taking any remedial action and it's come back as expected. The assumption being it was down to some kind of external contamination or sampling error.