Case definitions will allow an objective categorisation of individuals based on their disease status, which facilitates a camparison of  disease occurrence between populations, in different location and at different times.

A typical case definition includes:

  • clinical criteria and laboratory findings to characterise the disease,
  • a clear time period within which we count cases,
  • a precise identification (personal characteristics) of the population from which we count cases and its location.

Therefore: the disease, the time, the place and the person.

For example the case definition used in a school outbreak of Measles was: "A case patient was a child of School A, aged 5 to 15 years, with an illness characterised by a generalised rash lasting greater than or equal to 3 days AND a temperature greater than or equal to 38.3°C AND one of cough, coryza, or conjunctivitis; with onset of symptoms between 15 February and 28 March 28 2001."

The following table is a line listing of signs and symptoms of 11 children during the epidemic. It illustrates application of case definition criteria to count cases.

Child ID number

Age in years

Rash duration

Temperature in ° celsius

Cough

Coryza

Conjunctivitis

Onset date

Case

1

5

2

38.0

Yes

No

No

15/02

NO

2

12

0

39.0

Yes

Yes

No

17/02

NO

3

16

4

38.5

Yes

Yes

Yes

17/02

NO

4

14

5

38.7

Yes

No

Yes

18/02

YES

5

8

3

39.2

No

Yes

Yes

19/02

YES

6

9

2

38.0

Yes

Yes

Yes

19/02

NO

7

4

0

38.5

Yes

No

No

20/02

NO

8

13

0

37.0

Yes

Yes

No

21/02

NO

9

10

5

38.5

Yes

Yes

Yes

29/02

NO

10

11

3

39.0

No

No

Yes

27/02

YES

11

3

3

38.2

Yes

Yes

Yes

26/02

NO

Etc.

 

 

 

 

 

 

 

 

The table also demonstrates the difference between 'case' and 'patient'. It is clear that there are many more 'patients' in this table: in fact all people have symptoms of some illness. However, for the particular case definition that we have chosen here, only 3 count as a case. This can sometimes lead to heated discussions with clinicians, who want to know why their patient is not included as a case in the study. Such question is valid. Consider child number 6 in the table. The only reason that this child is not a case, is because there is no temperature over 38.5; is that a good justification to exclude this child as a case? The answer is: 'it depends'. It depends on the purpose of your study: what is it that you want to achieve?
The epidemiologist always needs to be able to justify why a certain case definition is chosen and what is the rationale behind each of the criteria.

An important question that we have to keep asking ourselves is:

- How much relevant information do I lose if the case definition is more specific or more sensitive?

During the stage of an analytical study, we want the case definition to be as specific AND sensitive as possible, to avoid misclassification (case / non-case), since misclassification biases the study results towards the null-hypothesis. In that sense, take another look at the table above: is it really justified that patient nr 3 is not counted as a case? This patient does not have the right age (5-15 years), however has all other elements of the case definition. A solution may be, to allow various levels of certainty in a case definition (possible, probable, confirmed).

Depending on where we are in the investigation, we need to be willing to take the responsibility to modify the case definition, in order to minimise bias. Though this is the ultimate responsibility of the lead investigator, different experts in the outbreak team may have relevant contributions to the discussion: epidemiologist, microbiologist, clinician, etc.