Case Definitions and Classification
A case definition is a standard set of criteria for deciding whether a person should be counted as having a particular disease for surveillance or investigation purposes. It usually combines clinical, laboratory, and epidemiological criteria and typically supports graded classification - confirmed, probable, and suspected - so that counts are consistent across people, places, and time.
Definition
A case definition is a standardised set of criteria - commonly clinical features, laboratory confirmation, and epidemiological linkage - used to determine whether an individual is counted as a case of a given condition for surveillance or investigation, often with tiered classification into confirmed, probable, and suspected cases.
Scope
This topic explains what a case definition is, why standardisation matters for comparability of counts, and how graded classification (confirmed, probable, suspected) balances sensitivity against specificity. It treats case definitions as a measurement and surveillance concept; it is not a diagnostic protocol for individual patient care.
Core questions
- What is a case definition and why must it be standardised?
- How do confirmed, probable, and suspected categories differ?
- How does a case definition trade sensitivity against specificity?
- How does changing a case definition affect counted incidence and comparability over time?
Key concepts
- Case definition
- Clinical, laboratory, and epidemiological criteria
- Confirmed, probable, and suspected classification
- Sensitivity versus specificity of a definition
- Comparability and standardisation of counts
- Effect of definition change on case counts
Mechanisms
A case definition fixes the numerator of disease frequency by specifying who counts as a case. By combining clinical, laboratory, and epidemiological criteria, it makes counts consistent across observers and settings. Tiered classification lets investigators include cases at different levels of certainty: a broad (sensitive) definition captures more true cases but admits more non-cases, while a narrow (specific) definition does the reverse. Because the definition determines what is counted, any change to it alters apparent incidence and threatens comparability over time, so its formulation is a deliberate measurement decision in surveillance and outbreak investigation (Reingold, 1998; Thacker & Berkelman, 1988; Giesecke, 2017).
Clinical relevance
Case definitions explain why surveillance counts and clinical diagnoses can diverge and why reported figures shift when definitions are revised. They are tools for consistent population-level counting and are described here as such; they are not intended as criteria for the diagnosis or management of an individual patient.
Epidemiology
Standardised case definitions are foundational to notifiable disease reporting and outbreak investigation, where investigators commonly establish an explicit definition before counting cases and may use graded categories to capture varying certainty (Reingold, 1998; Thacker & Berkelman, 1988).
Evidence & guidelines
The role and structure of case definitions are established in surveillance and field epidemiology methodology and textbooks; specific definitions for individual diseases are issued by public health authorities and are outside the scope of this reference entry (Thacker & Berkelman, 1988; Reingold, 1998; Giesecke, 2017).
History
As routine disease reporting matured during the twentieth century, the need for explicit, standardised criteria to make counts comparable across jurisdictions led to formal surveillance case definitions and tiered classification; the importance of establishing a case definition at the outset of investigation is emphasised in field epidemiology writing (Reingold, 1998; Thacker & Berkelman, 1988).
Key figures
- Arthur L. Reingold
- Stephen B. Thacker
- Ruth L. Berkelman
- Johan Giesecke
Related topics
Seminal works
- reingold-1998
- thacker-1988
Frequently asked questions
- What is a case definition?
- It is a standard set of criteria - usually clinical, laboratory, and epidemiological - used to decide whether a person is counted as a case of a disease for surveillance or investigation, so that counts are consistent.
- Why are cases classified as confirmed, probable, or suspected?
- Graded classification lets surveillance capture cases at different levels of certainty, balancing a sensitive definition that catches more true cases against a specific one that reduces misclassification.