Case to case study design

Case-to-case studies are types of case-control studies used when the disease of interest can be sub-classified into two or several groups that have specific risk factors. In a case-to-case study, cases with a particular sub-type of disease are compared to cases with another subtype. For example, during a listeriosis outbreak, cases with the outbreak sub-type would be compared to sporadic cases (the controls).

Some assumptions are made. Non-outbreak cases (the controls), if infected with the outbreak subtype, would have been classified as cases. They come from the same population, which gave rise to outbreak cases. They represent exposure (e.g. food consumption) in the source population for outbreak cases. This is probably the major issue. Are sporadic cases of listeriosis representing food consumption in the general population? This may not always be true. Non-epidemic cases may be more likely to be exposed than the overall source population. We may therefore underestimate the odds ratio.

Some advantages lie with case-to-case studies. Cases are readily available. Since all subjects in the study are sick, there also may be less differential recall between cases and controls.

Case-to-case studies may be convenient when information is available for the sub-class of cases used as controls. However, as in any case-control study, investigators need to be very cautious and verify that exposure in the control group accurately reflects exposure in the source population for cases.

FEM PAGE CONTRIBUTORS 2007

Editor

Masja Straetemans

Original Authors

Alain Moren

Jean Claude Desenclos

Marta Valenciano

Arnold Bosman

Contributors

Masja Straetemans