In epidemiology, what is a case definition?

• A. A protocol for calculating sample size.
• B. A standard set of criteria for classifying a person as having the disease.
• C. A method for adjusting for confounding.
• D. A plan for distributing vaccines in a population.

Answer: (B)

A case definition is a standardized set of criteria used to decide whether a person has a disease for surveillance and study. It ensures that across clinicians and researchers, cases are identified in the same way, so data are comparable over time and across places. The criteria often combine clinical signs and symptoms with laboratory evidence, and may include categories like suspected, probable, or confirmed depending on how much proof is needed. This concept hinges on balancing sensitivity (capturing true cases) and specificity (excluding non-cases). It’s distinct from things like calculating how many participants you need in a study, adjusting for confounding in analysis, or planning how vaccines are distributed.