May 23 is the compliance date for the National Provider Identifier (NPI) to be used exclusively for electronic health care claims under HIPAA.  Providers who do not use their assigned NPI after this date may find health insurers starting to reject and return electronic claims.  Although millions of NPI numbers have been issued, it is unclear how may providers are in compliance.  As a result, the next several weeks-to-months are likely to be bumpy as providers begin to find that claims they believe are compliant are rejected.  Some commentators have predicted that if the industry experiences severe problems starting over the Memorial Day Weekend, CMS might relax the deadline.  Health Data Management noted, however, that providers that get too many claim rejections may resubmit the claims on paper. That will enable providers to get paid, but slow the process considerably and adversely affect cash flow.