The correct answer is B, "The rate may be higher if the denominator is very small," as this providesthe most plausible explanation for the observed data in the annual report. According to the Certification Board of Infection Control and Epidemiology (CBIC) guidelines, the CAUTI rate is calculated as the number of CAUTIs per 1,000 catheter days, where catheter days serve as the denominator. The report indicates a dramatic decrease in urinary catheter days and a slight decrease in the number of CAUTIs, yet the overall CAUTI rate has not increased. This discrepancy can occur if the denominator (catheter days) becomes very small, which can inflate or destabilize the rate, potentially masking an actual increase in the infection risk per catheter day (CBIC Practice Analysis, 2022, Domain II: Surveillance and Epidemiologic Investigation, Competency 2.2 - Analyze surveillance data). A smaller denominator amplifies the impact of even a slight change in the number of infections, suggesting that the rate may be higher than expected or less reliable, necessitating further investigation.

Option A (the rate is incorrect and needs to be recalculated) assumes an error in the calculation without evidence, which is less specific than the denominator effect explanation. Option C (the rate is not affected by the number of catheter days) is incorrect because the CAUTI rate is directly influenced by the number of catheter days as the denominator; a decrease in catheter days should typically lower the rate if infections decrease proportionally, but the lack of an increase here suggests a calculation or interpretation issue. Option D (decreasing catheter days will not have an effect on decreasing CAUTI) contradicts evidence-based practice, as reducing catheter days is a proven strategy to lower CAUTI incidence, though the rate’s stability here indicates a potential statistical artifact.

The explanation focusing on the denominator aligns with CBIC’s emphasis on accurate surveillance and data analysis to guide infection prevention strategies, allowing the infection preventionist to advise administration on the need to review data trends or adjust monitoring methods (CBIC Practice Analysis, 2022, Domain II: Surveillance and Epidemiologic Investigation, Competency 2.5 - Use data to guide infection prevention and control strategies). This insight can prompt a deeper analysis to ensure the CAUTI rate reflects true infection risk.