Case report and case series
- Study design
One or several patients with something new or rare are described in detail. There is no comparison group: it raises the alarm but cannot prove a cause.
What it is
A case report describes one patient; a case series describes several with the same disease, exposure or event. They are the first warning of new diseases, unusual presentations and unexpected adverse drug effects. Because they have no comparison group, they generate hypotheses that analytic studies then test.
How it is built
- A clear case definition is set (clinical, laboratory, time and place criteria).
- Each patient's features are described: age, sex, exposures, course.
- Shared findings are summarised as counts and percentages.
- The discussion states which hypotheses they suggest and which study would test them.
Worked example
- Over 4 months, a hospital sees 9 adults with a rare liver injury; 7 of them drank the same herbal infusion.
- Share with the exposure: 7/9 = 0.778, about 78%.
- The key fact is missing: how many healthy people in the same town drink that infusion?
- If 75% of the town drinks it, 78% among the sick is barely above what is expected.
- If only 10% drink it, 78% is striking and the hypothesis gains strength.
- Reading: the series raises the alarm; a case-control study with healthy neighbours would test the suspicion.
Strengths and weaknesses
| Strengths | Weaknesses |
|---|---|
| Fast and cheap | No comparison group |
| Detects new diseases and rare events | Cannot measure association or causation |
| Describes the clinical picture in detail | Published cases tend to be the most striking ones |
| Generates hypotheses for analytic studies | Highly exposed to chance with few patients |
When to use it
- Many pharmacovigilance alerts start with case reports of a serious, rare adverse event.
- The first descriptions of an emerging disease are usually case series from one hospital or city.
Common pitfalls
- Concluding that an exposure causes the disease because most cases had it: without its frequency in healthy people there is nothing to compare.
- Calling a case series followed over time a cohort study: if everyone has the disease and there is no comparison, it is still descriptive.
Test yourself
Think of the answer, then tap the question to see it.
What does a case series lack that it would need to measure an association?
A comparison group: without the frequency of exposure in people without the disease, neither RR nor OR can be calculated.
What is a case report mainly for?
Describing something new or rare (an emerging disease, an unexpected adverse effect) and generating a hypothesis.
What design is the natural next step after a case series with a suspected exposure?
A case-control study, which compares exposure in patients and in comparable healthy people.
Connections
Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.
Sources: The design, applications, strengths and weaknesses of descriptive studies and ecological studies (Health Knowledge (Faculty of Public Health), 2018); Principles of Epidemiology | Lesson 1 - Section 6 (Centers for Disease Control and Prevention, 2012); Chapter 24: Including non-randomized studies on intervention effects (Cochrane, 2024).
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Representative model, for educational purposes only.