With the increasing popularity of privacy blockers like Ghostery, 88% seems like a pretty solid number. Since Analytics is mainly used for determining trend lines and making decisions on those trends, I wouldn't lose too much sleep over that discrepancy.
However, it could never hurt to run an audit on your implementation just to make sure you've got everything covered. Partners such as ObservePoint have specific tools to help you audit your site and make sure there aren't any Analytics holes.