Prevalence

Prevalence is a measure used in epidemiology to describe the proportion of a population that has a given disease, condition, or attribute at a particular time or over a specified period. It is commonly expressed as a percentage or per 1,000 individuals. There are several forms: point prevalence, the proportion with the condition at a specific moment; period prevalence, the proportion who have had the condition at any time during a defined period; and lifetime prevalence, the proportion who have ever had the condition by the time of assessment.

Calculation commonly relies on cross-sectional data or administrative records: prevalence equals the number of existing cases at time t divided by the total population at time t. For period prevalence, cases are counted if they have the condition at any time during the period. The measure is not a rate in the strict sense, but a proportion.

Prevalence is influenced by both the incidence of the condition and its average duration. In a simple steady-state scenario, prevalence ≈ incidence × average duration. Diseases with long duration or high incidence tend to exhibit higher prevalence, while acute, short-lived conditions may have low prevalence even when incidence is high.

Measurement challenges include defining a case, diagnostic criteria, underreporting, misclassification, and differences in population structure. Age standardization is often used to compare prevalence across populations.

Applications of prevalence include assessing the burden of disease, informing health planning and resource allocation, monitoring trends, and evaluating public health interventions.