LOS ANGELES — Rigorous cleaning practices don’t ensure that medical scopes are free of contamination, and many of these reusable devices have scratches and dents that could harbor blood, tissue and bacteria, a new study found.

The seven-month study, in the American Journal of Infection Control, found that 12 of 20 gastroscopes and colonoscopes examined tested positive for bacterial growth, even after being disinfected using the current guidelines or additional measures.

In addition, 17 of the scopes were pulled from use at the end of the study and returned to the manufacturer for repair due to serious defects. Photos in the study show numerous scratches and dents on the ends of the scopes that could trap organic material as well as stains, debris and residual fluid stuck inside scope channels.

The scopes were relatively new at the beginning of the study and all were manufactured by Tokyo-based Olympus Corp., the leading maker of gastrointestinal scopes used in the United States and worldwide.

“Physicians, other caregivers, hospitals and regulators should be paying keen attention to this issue, as patients have a right to assume that clean instruments are being used on them,” said Cori Ofstead, the study’s lead author and an epidemiologist in St. Paul, Minn.

This study and other outbreak reports suggest a broader problem affecting several types of scopes, which could put more patients at risk.