BarrieM,
It has been explained to me both here and by Medical professionals that the choice of Lab can be critical, and the choice of machine in that lab can also be critical. When the measurements are so sensitive even allowing for tolerances, for consistency you need to have your sample tested on the same machine in the same lab, unless it has been re-calibrated, in which case you start with "new levels" all over again.
Different machines, different makes of machine from different manufacturers, a dish that is not thoroughly cleaned or sterilised, would probably all affect your sample when tested?
Maybe someone with more expertise will come along and hopefully reassure you?
atb
dave