Colonies that seem to survive in an area of high mite investation may not be due to the 'resistance' quality of that colony. There are many instances where colonies do not collapse. On closer examination, you may find mite levels that exceed the upper limits of models that have been generated (e.g. 24,000 mites in a colony with 8 frames of bees).  From these colonies, the conclusion  they are still thriving may be because of the general health of the colony and disease has not been a contributing factor.
 
For any claim in tolerance, I would like to see evidence that there are no mites in the hive. If you don't use a diagnostic tool, maybe a trapping system might be in order to monitor the natural mortality of the mite (if there are any).
 
Daryl Wright
England