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.