Domain 2: Surveillance and Epidemiologic Investigation
Surveillance and Epidemiologic Investigation tests case definitions, rate calculation, surveillance systems, outbreak investigation, and basic study design. The exam expects you to use data consistently and to separate signal from surveillance artifact.
Core rules and concepts
- A surveillance case definition is applied uniformly; it may differ from a clinical definition and should not be changed mid-outbreak without explanation.
- Incidence is new cases divided by population at risk over time; prevalence is all existing cases at a point or period. Device-associated rates use device-days as the denominator.
- The standardized infection ratio (SIR) compares observed infections with predicted infections from a risk model; SIR < 1 means fewer than predicted, not necessarily zero risk.
- Outbreak steps: verify the diagnosis, establish a case definition, create a line list, describe person/place/time, generate hypotheses, test with an analytic study, and implement control.
Worked example
At a wedding, 30 of 60 people who ate chicken became ill, versus 5 of 30 who did not. Attack rates are 50% and 17%, and the relative risk is about 3.0. The reasoning is to compare attack rates by exposure, not overall illness, because the overall rate mixes exposed and unexposed guests. This descriptive result supports a chicken hypothesis and prompts culture of food and stool, but does not alone prove causation.
Traps
Using prevalence as incidence, changing denominators, comparing raw counts instead of rates, ignoring increased testing as a cause of more cases, and stopping at descriptive epidemiology without a control measure cost marks.