A California hospital is contacting some 300 morbidly obese patients after learning that they may have been exposed to hepatitis or HIV by poorly cleaned instruments used in stomach-reduction operations.

Administrators at Scripps Memorial Hospital in San Diego said on Wednesday the patients were asked to take blood tests after it was discovered that a nurse had failed to follow proper procedures for sterilizing the instruments.

“The risk (of infectious disease) is extremely low but to be safe and take every precaution we are having blood drawn and tested, looking for HIV, hepatitis B and hepatitis C,” Scripps spokesman Don Stanziano said.

Stanziano said state health officials were overseeing an investigation into the nurse’s failure to properly sterilize an instrument called a gastroscope.

The gastroscope, which is used in gastric bypass procedures, requires a different sterilization process from most instruments because it cannot be exposed to high temperatures.

He said results of the blood tests would be available in two to three weeks.