RALEIGH, N.C. — Duke University Health System refuses to tell patients and their doctors what was in hydraulic fluid mistakenly used to clean operating equipment.

In January, Raleigh Duke Health and Durham Regional Hospital sent letters to thousands of patients about a "possible problem."

The hospitals used hydraulic fluid to clean surgical instruments.

Duke blames the mix-up on contractors working on hospital elevators.

Several patients are worried about possible health issues and want to know exactly what was in the fluid.

Hospital officials said they could not give the information to anyone while they investigate the mistake, but they did say that infection rates have not increased as a result.