January 8th, 2015
The GP modifier rule is defined on page 36 of this link: http://www.cms.gov/Regulations-and-guidance/Guidance/Manuals/downloads/clm104c05.pdf
In short, providers are supposed to append the GP modifier to any physical therapy type services when submitting them to Medicare. The result will be the same...the service will deny because therapies are not covered by Medicare. Medicare uses the modifier in their claims processing/recognition system for statistics and monitoring purposes. Basically, they can see who's using therapies and how often therapies are being done.
Some Major Medical payers have followed suit and require the GP on physical therapy services submitted to them. Most aren't requiring this yet and there isn't a list. Providers will have to either check the coverage and billing guideline for individual payers or find out by trial and error if it is required by a particular payer. It isn't a frequent occurrence at this time.