Case to case studies are types of case control studies used when the disease of interest can be sub-classified in two or several groups that have specific risk factors. In a case to case study cases with a particular sub-type of a 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 a convenient design 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 reflects accurately exposure in the source population for cases.