The Person / Place / Time triad
Step 4 of the checklist (the working case definition) is the foundation everything else is built on. A good case definition always answers three questions, known as the person, place, and time triad:
CDC lists it as clinical criteria (symptoms, lab results) plus limits on person, place and time. On a test, name the clinical criteria as well as person, place and time.
Who counts as a case? Set clinical criteria (symptoms, lab results) and sometimes limits like age group or occupation.
Where does a case have to be located? A school, a restaurant's customers, a specific ZIP code or region.
When did illness onset have to occur? A specific date range tied to the suspected exposure window.
Twenty students at Cedarwood Middle School's spring banquet report stomach illness the same evening. A working case definition might read:
Case: Any student or staff member who attended the Cedarwood Middle School spring banquet on May 3 (place) and developed vomiting or diarrhea (person: clinical criteria) within 12 hours after the event (time).
Notice the definition doesn't try to name the cause yet: it just draws a clear, repeatable line around who counts.
Leave the suspected exposure out. Don't write "and ate the potato salad" into a case definition. The whole point of the study that follows is to compare the ill and the well on what they ate; if eating the salad were part of being a case, every case would have eaten it by definition, and the salad would look guilty no matter what.
Usually a positive lab test for the agent. Each disease's case definition sets the exact rules.
The typical signs and symptoms (often plus a link to the outbreak), but no lab confirmation yet: the test is pending or wasn't done.
Only some of the symptoms, not the full clinical picture, and no lab confirmation.
Doesn't meet the case definition, or a lab test found a different agent or ruled the disease out.
Confirmed for some diseases (like Salmonella) but not others (measles needs a fever and rash too). Check that disease's case definition.
A lab result for a different pathogen means not a case, even though the person looks sick.
An imported case got the disease elsewhere, like a traveler who got measles abroad in the 21 days before their rash. It can start a local outbreak, but it isn't counted as locally acquired.
For some diseases a link to a confirmed case is enough. Under the measles definition, someone with the typical illness who had contact with a lab-confirmed case is confirmed, even with no test of their own.
Q1A case definition says: "Anyone who attended Lincoln Elementary's book fair between March 4 and 6 and developed a rash within 48 hours." Which part is the "place"?
Q2Person, place or time? Sort each part of these case definitions.
Q3A guest at the reunion has all the typical symptoms, but no lab test was done. Using the confirmed / probable / possible tiers, how is this guest classified?