Prevalence is defined as the total existing cases at a given time divided by the population; it can increase even if incidence remains constant. Which option best reflects this definition and scenario?

Prevalence is defined as the total existing cases at a given time divided by the population; it can increase even if incidence remains constant. Which option best reflects this definition and scenario?

• A. Prevalence = the number of new cases in a population during a specified time period; it increases when incidence rises.
• B. Prevalence = total existing cases at a given time divided by the population; it can increase with longer disease duration or decreased mortality/remission.
• C. Prevalence = incidence rate multiplied by average duration of disease.
• D. Prevalence = the proportion of the population currently at risk of disease.

Answer: (B)

Prevalence measures how many people in a population are living with a disease at a given moment, expressed as existing cases divided by the population. It can rise even if incidence stays the same because people with the disease may live longer or have reduced mortality/remission, keeping them in the pool of existing cases for a longer time. That’s why the statement that prevalence is total existing cases divided by the population and can increase with longer disease duration or decreased mortality/remission best captures the concept. In contrast, describing prevalence as new cases over time reflects incidence, not prevalence; a prevalence–incidence–duration relationship is a useful rule-of-thumb but depends on steady-state assumptions; and prevalence is not the same as the proportion at risk of disease.

Prevalence measures how many people in a population are living with a disease at a given moment, expressed as existing cases divided by the population. It can rise even if incidence stays the same because people with the disease may live longer or have reduced mortality/remission, keeping them in the pool of existing cases for a longer time. That’s why the statement that prevalence is total existing cases divided by the population and can increase with longer disease duration or decreased mortality/remission best captures the concept. In contrast, describing prevalence as new cases over time reflects incidence, not prevalence; a prevalence–incidence–duration relationship is a useful rule-of-thumb but depends on steady-state assumptions; and prevalence is not the same as the proportion at risk of disease.