DURHAM, N.C. — Duke University Health System sent out another letter this week to assure patients it has not closed the door on a medical mix-up affecting nearly 4,000 patients.

In the latest letter, the fourth one since January, the hospital system outlines several major steps it is taking after last year's medical mix-up in which surgical instruments tainted by hydraulic fluid were mistakenly used in the cleaning process.

Duke Health told patients it is considering a round of animal tests that would mimic exposure. It has launched a new Web site to answer patients' questions and has also set up a new patient advocacy board to open lines of communication and help the hospital deal more effectively with problems.

Patients will also be tracked through doctors' reports, hot line calls and health records.

Last year, doctors performed thousands of surgeries with the instruments. Despite the mix-up, Duke Health insists the surgical tools were sterile and that the risk of infection was low. Doctors admit they are not sure about long-term affects.

Last month, the hospital set up a hot line that concerned patients could call to get any questions they may have answered, but officials said in the letter they are receiving very few calls.

One patient told WRAL that while she is happy Duke Health is making the effort, she feels it is still coming up short. She feels a more formal investigation by an independent source would be a better way to go.

And she said the level of trust between the hospital system and the patients has been badly damaged.