Developing a control definition

Controls should have the following characteristics.

• be representative of the exposure distribution in the source population
• have an equal chance of being identified as cases if they had the disease under study
• have the same exclusion and restriction criteria as cases

One way to develop a control definition is to consider the case definition proposed for the investigation. The following case definition might be used in a hypothetical outbreak of E coli O157, mainly among young children, mainly in London, during June 2010.

Case

Resident of London aged under 10 years with a faecal isolate of E. coli O157 during June 2010.

Exclusion

Travel abroad in the week before the onset of illness.

1. The source population for cases is residents of London in June 2010 aged under 10 years. Controls should be representative of this source population.

1. Since E. coli is a severe infection of children, we would expect all children in London to have a similarly high chance of being detected as cases if they had this infection. However, there may be variations in the proportion of cases diagnosed by geographical area through variations in factors such as health-seeking behaviour, primary care sampling, and diagnostic facilities. This may introduce a selection bias when choosing controls, as it will be difficult to identify this source population. This bias will not matter unless the proportion exposed differs between cases identified for our study and those undetected cases.

1. In this definition, cases have been excluded if they travelled abroad the week before illness onset. An equivalent suitable exclusion period for controls might be travelling abroad in the week before the interview. However, if cases mostly arise during school term and if controls are interviewed in the summer holidays, some controls may be excluded unnecessarily. Another option might be to exclude those who travelled abroad in June. Or, if individually matched on potential time of exposure, travel exclusion could be restricted to the dates of the week before the onset of illness in the matched case.

A suitable control definition might be:

Control

Resident of London aged under 10 years during June 2010.

Exclusion

Travel abroad in the week before the interview.

FEM PAGE CONTRIBUTORS 2007

Editor

Naomi Boxall

Original Author

Alain Moren

Contributors

Sabrina Bacci

Lisa Lazareck

Naomi Boxall