03
Building a Case Definition
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:
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Person
Who counts as a case? Set clinical criteria (symptoms, lab results) and sometimes limits like age group or occupation.
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Place
Where does a case have to be located? A school, a restaurant's customers, a specific ZIP code or region.
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Time
When did illness onset have to occur? A specific date range tied to the suspected exposure window.
🍔 Try It: Writing a Case Definition
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.
Case definitions often tighten over time. Investigators frequently start broad (to catch every possible case) and then split results into confirmed, probable, and suspect categories as lab results come back, narrowing the definition as evidence accumulates. A broad definition is more sensitive (it misses fewer real cases) but less specific (it lets in more people who aren't really cases); adding lab confirmation or stricter symptoms does the reverse. Lab criteria are included when a test is available, but they aren't required: many working definitions are clinical only.
Sorting people into case tiers
Confirmed
Usually a positive lab test for the agent. Each disease's case definition sets the exact rules.
Probable
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.
Possible (suspect)
Only some of the symptoms, not the full clinical picture, and no lab confirmation.
Not a case
Doesn't meet the case definition, or a lab test found a different agent or ruled the disease out.
Two edge cases tests love: (1) Someone with no symptoms but a positive lab test is usually still confirmed: for many diseases a positive lab test confirms a case even without symptoms, but each disease's case definition sets the exact rules. (2) Someone with every symptom but a lab result for a different pathogen is not a case, even though they look sick.
(3) An imported case caught the disease somewhere else, like a traveler who got measles abroad in the 21 days before their rash. An imported case can start a local outbreak, but it isn't counted as locally acquired.
TryClassify: (a) rash, fever and cough, lab pending; (b) cough only, no test; (c) no symptoms, lab positive; (d) rash, fever and cough, lab positive for a different virus.
(a) Probable. (b) Possible. (c) Confirmed. (d) Not a case.
✓ Check Yourself
✓ Complete
Q5A case definition says: "Anyone who attended Lincoln Elementary's book fair between March 4–6 and developed a rash within 48 hours." Which part represents "place"?
Lincoln Elementary's book fair. The dates are "time" and the rash is the clinical ("person") criterion.