By Ahmad Jamil Malik

CASE CONTROL STUDY

A type of retrospective study (backward-looking) in which two groups with different known outcomes are compared based on a supposed “risk factor”

In this type of study, we compare the subjects who have the condition/outcome  (the “cases”) with the subjects who do not have the condition/outcome (the “controls”)

The “risk factor” is something that happened in the past, probably before occurrence of the disease and it has increased the chances of disease development.

The basic design of case control study looks like this:-

In the case control study, the cases are selected mainly from their outcome status, apart from that it could be the date of diagnosis of their disease, the cases of a certain geographical location where the disease is prevalent, e.g. in case of cancer patients, it could a be a certain histological character in their biopsy. Newly diagnoses cases are preferred to the prevalent ones while their selection.

During the selection of the controls, it is preferred to select them from the same population as that of the cases with the same confounding variables. It can be controls from same institution or same hospital where the cases were admitted.

An example of case control study is that of, “Clear cell carcinoma of vagina with use of Diethylstilbestrol”. Diethylstilbestrol was prescribed between 1938-1971 by physicians in order to prevent any miscarriage. Later on, it was found to be teratogenic and carcinogenic form of synthetic estrogen so it’s use was stopped. Diethylstilbestrol acted as a precursor of cancer in the children of those women who had documented use of Diethylstilbestrol causing  vaginal or cervical dysplasia, cervical intraepithelial neoplasia, and squamous intraepithelial lesions. In this study, risk factors related to DES were studied in 156 patients whose mothers (cases) had documented exposure to DES. 1,848 DES-exposed women of similar age without cancer (controls). Relative risks were higher for those whose mother began DES prior to the twelfth week of pregnancy. Maternal history of at-least one spontaneous abortion also elevated the risk.

Advantages of case control study include:

• Feasible when disease occurs rarely eg. mesothelioma.
• Requires comparatively fewer subjects
• Rapid and inexpensive
• No risk to subjects

Disadvantages of case control study include:

• Does not distinguish between cause and associated factors
• Incidence cannot be measured
• People’s recall of their past behavior may lead to “recall bias”

Multifactorial case control study:

The common form of case-control study addresses one main factor or attribute at a time. It is possible, however, to investigate several exposure factors in the same study using the multi factorial case control study.