WASHINGTON (AP) -- Five years after being told to look at taking Social Security numbers off Medicare cards, Medicare officials say they still need six more months to figure out how much it will cost.

At a sometimes-tense House hearing, Medicare Chief Information Officer Tony Trenkle assured lawmakers that the issue is a priority but couldn't offer a timetable for when the change might be made.

Congressional auditors said in a report Wednesday that an earlier cost estimate of $800 million to $845 million was faulty, partly because of insufficient and inconsistent data. The Government Accountability office estimated as many as 48 million Medicare beneficiaries risk having their identity stolen because their Social Security numbers are displayed on Medicare cards required by most physicians' offices and other health care providers.