DURHAM, N.C. — Duke University Health System plans to bring in an outside organization to track nearly 4,000 patients involved in a medical mix-up.

Last year, surgical instruments at two Duke Health-affiliated hospitals were mistakenly washed in used hydraulic fluid during a cleaning process. While Duke Health insists the tools were sterile and risk for patient infection low, doctors admit they do not know about long-term effects.

With patient tracking, the health system hopes to follow patients' progress for as long as three years. A spokesman for the health care giant says bringing in an independent company will add scientific validity to the work and help ease patients' concerns.

In January, the hospital system sent letters informing patients of the medical mix-up. Since then, many affected patients complained Duke Health's response lacked urgency and compassion.

When the story became a frequent headline in the news, Duke Health administrators changed strategy and sent more letters. They also launched a telephone hotline for patients to call, a patient consultation center and an informational Web site to keep those affected updated on the latest news and information.