It's no joke -- effective on April Fool's Day, your Medicare claims must be in the new HIPAA-compliant 5010 format. The "vast majority" of provider claims already are being sent to Medicare in 5010 format, CMS says in an e-mail to providers. "Given these favorable results, we are taking the next step towards full implementation of 5010 in Medicare Fee-For-Service." What that means: If you submit a claim in the old 4010 format after March 31, your Medicare contractor will return it as unprocessable. "Failure to submit 5010 formatting will result in your claim being unpaid/denied," CMS stresses.