Waterline CFU Standard — MFDS Part 1 MCQ

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ModerateMicrobiologyWaterline CFU StandardMFDS Part 1

A dental unit waterline sample is tested for its aerobic colony count at 22°C. According to current UK microbiological guidance, which result is classified as acceptable, rather than satisfactory or undesirable?

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Correct answer: C — 100–200 CFU/mL

Explanation lettering: E = shown as A · D = shown as B · A = shown as C · B = shown as D · C = shown as E

The correct answer is A. Current UKHSA guidance classifies an aerobic colony count at 22°C of 100–200 CFU/mL in dental unit waterline output as acceptable; appropriate waterline controls should be confirmed. A count below 100 CFU/mL is classified as satisfactory, so option B represents the preferred result rather than the specifically requested acceptable category. Any count above 200 CFU/mL is undesirable and should prompt discussion with a microbiologist, investigation of the cause and corrective action. Therefore, options C, D and E are all above the acceptable upper limit. Sterility is not the routine microbiological standard for dental unit waterline output.

Reference: UK Health Security Agency. Examining food, water and environmental samples from healthcare environments: microbiological guidelines, Table 8, updated 31 March 2025. https://assets.publishing.service.gov.uk/media/67e673f78ac59d1882eadda1/Hospital_FWE_Microbiology_Guidelines.pdf