When billing PET to Medicare we use the PI or PS modifier as required by Medicare as well as either a TC or 26 modifier depending on whether the service was technical or professional. Recently we've started getting rejections for "inappropriate/invalid modifier", and were wondering if the order in which you list the modifiers on the claim would have anything to do with the rejection? I've been putting the TC or 26 first on the claim. Should the PI/PS go in the first slot? Has anyone else been getting rejections like this?