The complaint alleges that UnitedHealth Group and its subsidiary NaviHealth use an algorithm called nH Predict to systematically deny Medicare Advantage patients rehabilitation care, knowing the algorithm has a high error rate. It alleges a 90 percent error rate, based on the share of denials reversed on appeal, and that only a tiny share of patients appeal. UnitedHealth said the tool is a guide and not used to make coverage determinations, and that the lawsuit has no merit.