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Dialysis would not be covered by any of the vistors' insurance policies. The maintenance, regular testing, and existing medication refills are not covered for any pre-existing conditions.
Travel medical insurance plans are designed to cover those traveling outside of their home countries for any new, unexpected sicknesses or injuries that may happen while they are abroad. For an explanation of what a pre-existing condition is and why they are not covered, see: http://www.visitorscoverage.com/travel/pre-existing-conditions-travel-insurance/
There are a few plans that do offer coverage for an acute onset of pre-existing conditions. You may read more about this particular type of coverage, here: http://www.visitorscoverage.com/acute-onset-of-pre-existing-conditions/