Categories of Descriptive Epidemiology
Case Reports and Case Series
Case Reports
A case report is a detailed description of disease occurrence in a single person. Unusual features of the case may suggest a new hypothesis about the causes or mechanisms of disease.
Example: Acquired Immunodeficiency in an Infant; Possible Transmission by Means of Blood Products
In April 1983 it had not yet been shown that AIDS could be transmitted by blood or blood products. An infant born with Rh incompatibility; required blood products from 18 donors over 8 weeks and subsequently developed unusual recurrent infections with opportunistic agents such as Candida. The infant's T cell count was low, suggesting AIDS. There was no family history of immunodeficiency, but one of the blood donors was found to have died of AIDS. This led the investigators to hypothesize that AIDS could be transmitted by blood transfusion.
Case Series
A case series is a report on the characteristics of a group of subjects who all have a particular disease or condition. Common features among the group may suggest hypotheses about disease causation. Note that the "series" may be small (as in the example below) or it may be large (hundreds or thousands of "cases"). However, the chief limitation is that there is no comparison group. Consequently, common features may suggest hypotheses, but these need to be tested with some sort of analytical study before an association can be accepted as valid.
Example: Discovery of HIV in the United States
In 1980 -1981 four previously healthy young men were diagnosed with Pneumocystis carinii pneumonia, an unusual "opportunistic" infection that had only been seen in immune compromised people with hereditary disorders or in people with immune compromise due to chemotherapy. The medical histories didn't suggest any preexisting immunodeficiency, but all four men had decreased immune responses and low T cell counts. These unusual infections suggested the possibility of a previously unknown disease. It was noted that all four men were sexually active homosexuals, and in the case series which was published in the New England Journal of Medicine the authors speculated that the immune dysfunction was due to a sexually transmitted infectious agent.
This was an extraordinarily important case series (a detailed description of characteristics of a series of people who all have the same disease) that suggested that this new syndrome was associated with sexual activity in male homosexuals. Alerting the medical establishment and proposing a hypothesis was an important milestone in the AIDS epidemic.
Test Yourself
Oral Contraceptives and Hepatocellular Carcinoma?
There had been a number of case reports of liver cancers in young women taking oral contraceptives. A study was undertaken by contacting all of the cancer registries collaborating with the American College of Surgeons. The investigators wanted to collect information on as many of these rare liver tumors as possible across the US.
Table - Oral Contraceptive Use Among Women Who Developed Liver Cancer
| Oral Contraceptive Use |
Age 16-25 yrs. |
Age 26-35 yrs. |
Age 36-45 yrs. |
|
Yes |
31% |
43% |
22% |
|
No |
20% |
10% |
29% |
|
Unknown |
49% |
48% |
49% |
What conclusions can you draw from these data regarding a possible increased risk of liver cancer in woman taking oral contraceptives? Think about it before you look at the answer.
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Key Concept: The key to identifying a case series is that all of the subjects included in the study have the primary disease or outcome of interest. For example, an article reported on 239 people who got bird flu. The article might present tables and graphs that gave information about their age, occupation, where they lived, whether they lived or died, etc., but basically it is a detailed description of the characteristics and outcomes in a group of people who all had the same disease.
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Strengths of Case Reports and Case Series
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Limitations of Case Reports and Case Series
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