To calculate the surgical-site infection attack rate for CABGs, what should be used as the denominator?

• A. Number of infections
• B. Total number of CABG surgeries
• C. Number of surgeons
• D. Number of hospitals involved in the study

Answer: (B)

Surgical-site infection attack rate uses the number of patients who could potentially develop an infection—the population at risk—measured by the total number of CABG procedures performed. The rate is the number of infections divided by the total CABG surgeries in the period, often expressed per 100 procedures. The infection count serves as the numerator, while the denominator must reflect how many surgeries were actually done, ensuring the rate meaningfully represents risk per procedure. The other options don’t define the at-risk population: infections alone is the numerator, and the number of surgeons or hospitals does not correspond to how many patients were exposed to the risk of SSI.