Case Reports and Case Series: What They Can and Cannot Tell You

Case reports sit at the bottom of most evidence hierarchies, yet they have a long and distinguished history in medicine and other fields. The first descriptions of AIDS, the thalidomide disaster and many rare diseases came from careful observations of individual patients. Case reports and case series remain the fastest way to signal new phenomena, unexpected harms and unusual presentations. They are also easy to misread. A striking story about one patient can feel more persuasive than a large trial, even though it cannot establish cause and effect. This guide explains what case reports and case series are, why they matter, how to appraise them, how to use them in literature reviews and how a source-linked assistant such as Yila can help you collect and compare individual reports while keeping each detail tied to its original source.

Definitions

A case report describes a single patient or event in detail: the presentation, investigations, diagnosis, treatment and outcome. A case series describes a group of patients with a similar condition, exposure or treatment, usually without a comparison group. The boundary between them is not sharp; a series of three patients is sometimes called a case report.

Case series should be distinguished from cohort studies. A cohort study follows people defined by exposure and compares outcomes between exposed and unexposed groups. A case series typically selects people because they have an outcome, or because they received a treatment, and describes them without comparison. Some studies labelled as case series are really small cohort studies, and vice versa, so look at the design rather than the label.

Why case reports matter

Case reports serve several purposes. They identify new diseases and syndromes. They detect rare adverse effects of drugs and devices that trials, with limited size and follow-up, cannot capture. They describe unusual presentations of known conditions, helping clinicians recognise them. They document novel treatments or diagnostic approaches, generating hypotheses for formal study. And they provide educational value, illustrating clinical reasoning in a concrete way.

Pharmacovigilance systems depend heavily on individual reports. A cluster of similar reports can trigger investigation, label changes or withdrawal of a product. Many safety signals were first noticed by clinicians who took the time to write up an unexpected event.

What they cannot do

Case reports and case series cannot establish causation or estimate how often something happens. Without a comparison group, it is impossible to know whether an outcome would have occurred anyway. A patient who improved after a new treatment might have improved without it; a patient who developed a complication after a drug might have developed it regardless.

They also cannot estimate frequency, because the number of people at risk is unknown. Ten reports of a side effect could reflect ten cases among a hundred users or among ten million. And they are subject to strong publication bias: dramatic, novel or positive cases are more likely to be written up and accepted than ordinary ones.

Appraising a case report

Despite their limitations, case reports vary widely in quality. Several questions help appraise them. Is the patient described in enough detail, including demographics, history, comorbidities and medications? Are the diagnostic tests and criteria clear, so that you can judge whether the diagnosis is secure? Is the timeline of exposure, intervention and outcome clearly presented? Were alternative explanations considered and ruled out? Is the follow-up long enough to know the outcome?

For suspected adverse drug reactions, structured causality assessments such as the Naranjo algorithm or the WHO-UMC system consider factors such as temporal relationship, dechallenge and rechallenge, alternative causes and previous reports. A report in which the reaction resolved when the drug was stopped and recurred when it was restarted is far more persuasive than one with a single temporal association.

The CARE guidelines provide a checklist for reporting case reports, covering the title, keywords, abstract, introduction, patient information, clinical findings, timeline, diagnostic assessment, interventions, follow-up, discussion, patient perspective and informed consent. Using CARE as a guide to what should be present helps you spot gaps.

Appraising a case series

For case series, additional questions arise. How were cases identified, and were they consecutive or selected? Consecutive inclusion reduces selection bias. Were inclusion criteria clear and applied consistently? Were all patients followed for an adequate period, and were outcomes measured in the same way? Are results reported for every patient, including those with poor outcomes?

Tools such as the JBI critical appraisal checklist for case series and the framework proposed by Murad and colleagues help structure these judgements. They focus on selection, ascertainment, causality and reporting.

Using case reports in literature reviews

Case reports are rarely included in systematic reviews of treatment effects, where trials and comparative observational studies are preferred. But they play important roles in other kinds of reviews. Reviews of rare conditions, where trials are impossible, may rely largely on case reports and series. Reviews of adverse effects often include them to capture rare harms. Scoping reviews and reviews describing clinical features may synthesise many case reports to build a picture of presentation and outcomes.

Synthesising case reports requires care. Data are heterogeneous and incomplete, and publication bias is severe. Individual patient data can sometimes be extracted from each report and pooled descriptively, providing summaries of features, treatments and outcomes. Such summaries should be presented as descriptive, not as estimates of frequency or effect. Duplicate reporting is a particular risk: the same patient may appear in a case report, a later case series and a registry analysis.

Practical workflow

When reviewing many case reports, a consistent extraction form is essential. Record patient characteristics, exposure details, timeline, diagnostic criteria, interventions, outcomes and causality assessment for each report. A source-linked assistant can help by summarising each report with links to the passages containing these details, speeding extraction while allowing you to verify each value. Duplicate detection benefits from careful comparison of patient characteristics, dates and institutions across reports.

Writing a good case report

If you are writing a case report, choose a case that offers genuine new information: a previously undescribed association, an unusual presentation with educational value or a novel management approach. Obtain informed consent from the patient. Follow the CARE checklist. Present the timeline clearly, describe your reasoning and the alternatives you considered, and be honest about uncertainty. Avoid overstating causal claims, and suggest what further research would be needed to confirm your observations.

Conclusion

Case reports and case series are valuable for detecting new phenomena, rare harms and unusual presentations, and for generating hypotheses. They cannot establish causation or frequency, and they are prone to selection and publication bias. Appraised carefully and synthesised appropriately, they contribute meaningfully to the evidence base, particularly where other designs are impossible.