Bosses at Pinderfields Hospital in Wakefield contacted 25 patients as a precautionary measure after they had endoscopy examinations.
The procedure uses a fibre optic tube connected to a camera to look inside the body.
The patients had all had a nasal endoscopy and were called in for further tests to check for blood-borne viruses.
Mid Yorkshire Hospitals NHS Trust said 18 had so far come forward and all their tests results were negative.
Director of nursing David Melia said: “Through our internal controls we identified an issue with the cleaning process of some of our nasal endoscopy equipment.
“Following advice from national experts at Public Health England we were advised that some of our patients who have had a nasal endoscopy procedure will need a follow-up blood test to rule out the very low risk of acquiring a blood borne virus.
“Therefore we have written to 25 patients advising them of the need to have a blood test.
“We would like to emphasise to our patients that the risk is extremely low and this is a precautionary step only.
“So far 18 of the 25 have come forward and we can confirm their results are all negative.”