Patients and providers who say insurers low-balled reimbursement paid under employee benefit plans may be looking for new arguments in this often-thorny legal area.

A federal court earlier this month decided that a group of out-of-network patients and providers could not form a class to sue Aetna for alleged underpayments of their claims. The decision was the most recent setback in a series of cases on behalf of plaintiffs suing over the Ingenix database, which allegedly skewed data to calculate...