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# Case definition

- Concept

A case definition is the fixed rule that decides who counts as a case in an investigation or in surveillance.

Updated Nov 13, 2026

## Key points

- Suspected case: meets clinical criteria only; probable: also has an epidemiological link or a presumptive test; confirmed: has laboratory confirmation (or a link, depending on the event).
- Every definition includes clinical criteria and restrictions of person, place and time, but never the exposure under study.
- A broad definition is more sensitive (misses few cases) and less specific (includes more false cases); a strict one does the opposite.
- Early in an outbreak a broad definition helps find cases; for the analysis a strict one avoids diluting the association.
- If the definition changes over time, the number of cases changes even when the disease does not: a numerator problem.

On this page

- The basics Key points
- What it is
- Structure
- Worked example
- Tiers of a case definition
- Markers
- In practice
- Common pitfalls
- Memory aid

- Study Test yourself

- Connections Connections

## What it is

A case definition is a standard set of clinical, laboratory, person, place and time criteria that decides whether someone is counted as a case. Thanks to it, everyone looking for cases counts the same thing, and rates can be compared across places and years. It is usually tiered into suspected, probable and confirmed cases, and how broad it is changes with the phase of the work.

## Structure

- Clinical criteria: signs and symptoms with a clear threshold (for example, fever plus rash).
- Laboratory criteria: which test confirms and which result counts.
- Person: residents, workers or people who attended an event.
- Place: a defined municipality, neighbourhood, institution or event.
- Time: date of symptom onset within a set period.

## Worked example

- Data (made up): an outbreak of fever with rash in a municipality; after the outbreak the laboratory confirms 52 true cases.
- Broad definition (fever plus rash since 1 March): 80 people meet it, of whom 48 are confirmed.
- Sensitivity of the broad definition: 48 / 52 = 0.923, that is, ≈ 92 % of the true cases.
- Share of true cases among those counted: 48 / 80 = 0.60, that is, 60 %; 32 people counted were not cases.
- Strict definition (also contact with a confirmed case): 40 people meet it, of whom 36 are confirmed.
- Strict sensitivity: 36 / 52 = 0.692 (≈ 69 %); true cases among those counted: 36 / 40 = 0.90 (90 %).
- Reading: the broad one is for finding cases and missing none; the strict one is for analysing risk factors with fewer false cases.

## Tiers of a case definition

| Tier | What it requires | Advantage | Limitation |
| --- | --- | --- | --- |
| Suspected | Clinical criteria only | Captures almost every case and triggers a fast response | Includes many people who do not have the disease |
| Probable | Clinical picture plus an epidemiological link or presumptive test | Balances finding and precision | Depends on the quality of contact tracing |
| Confirmed | Laboratory confirmation | Highest specificity | Misses cases with no sample or a late sample |

## Markers

- Suspected
- Probable
- Confirmed
- Epidemiological link
- Numerator

## In practice

- In Colombia the INS publishes surveillance forms and protocols for each event with its case definitions; doctors notify by that definition, not by impression.
- In a new outbreak the definition is written early and updated with a date and version when laboratory data arrive.
- If a case changes category (from suspected to ruled out), it is reclassified in the system so the count is not inflated.

## Common pitfalls

- Putting the suspected exposure into the definition (for example, ‘ate at the restaurant’): it creates a false association.
- Comparing rates for two years in which the definition changed, without noticing.
- Using only confirmed cases when laboratory access differs between regions: regions with fewer laboratories are underestimated.

## Memory aid

‘Suspect, link, prove’: each tier asks for more evidence and admits fewer false cases, but also loses a few true ones.

## Test yourself

Think of the answer, then tap the question to see it.

## What separates a probable case from a suspected one?

A probable case meets the clinical criteria and also has an epidemiological link to a confirmed case or a presumptive test; a suspected case meets the clinical picture only.

## Why is the exposure under study left out of the case definition?

Because every case would then be exposed by definition, and the association between exposure and disease would be artificial.

## What happens to the sensitivity and specificity of a definition when it is made stricter?

Sensitivity falls (true cases are missed) and specificity rises (fewer people without the disease are counted).

## What is a numerator problem in surveillance?

A change in the number of cases counted that does not reflect a real change in the disease, for example because the case definition or the way of notifying changed.

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## Connections

## Related 6

- [Outbreak investigation](https://durgical.com/en/epidemiology/outbreak-investigation/)
- [Public health surveillance](https://durgical.com/en/epidemiology/public-health-surveillance/)
- [Sensitivity](https://durgical.com/en/epidemiology/sensitivity/)
- [Specificity](https://durgical.com/en/epidemiology/specificity/)
- [Notifiable events in Colombia](https://durgical.com/en/epidemiology/notifiable-events-in-colombia/)
- [SIVIGILA: public health surveillance in Colombia](https://durgical.com/en/epidemiology/sivigila-public-health-surveillance-in-colombia/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Principles of Epidemiology: Lesson 6, Section 2](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson6/section2.html) (Centers for Disease Control and Prevention, 2012); [Fichas y Protocolos | INS](https://www.ins.gov.co/buscador-eventos/Paginas/Fichas-y-Protocolos.aspx) (Instituto Nacional de Salud (INS), Colombia, 2026).

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Representative model, for educational purposes only.

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