DURHAM, N.C. — Lawyers for Duke University Health System and attorneys for several patients, who may have been affected by a chemical mix-up, formalized a deal Monday that would let affected patients finally know what was in used hydraulic fluid that was used to accidentally wash surgical instruments.

Last fall, doctors at two Duke Health-affiliated hospitals used the surgical tools on close to 4,000 patients. Since then, many patients have complained of side effects and slow recoveries and wonder if the mix-up is to blame.

For months, attorney David Stradley said he has asked for a sample of the liquid to test independently. Lawyers for Duke said Stradley would have it by the end of the week.

"We just got no response for four months, and we're happy to have it," Stradley said.

Many have accused the hospital system of being slow to respond to the mistake, but Duke Health's attorney, Charles Holton, said his client was just being thorough.

Duke said it would also hand over all the tests results it has already received regarding the used hydraulic fluid.

The health system's attorney said officials are encouraged by what they have seen.

Patients should get letters this week regarding the results.

The health care system maintains, despite the mix-up, the surgical instruments were sterile.