Prevalence & Incidence
In normal everyday conversations incidence and prevalence are used interchangeably. That is because broadly speaking incidence and prevalence both measure the frequency of a disease or outcome. In most situations Incidence & prevalence are also directly proportional. However, there are a few clear differences between the two measures that the test writers love to write questions about.
- Prevalence = number of Total Existing cases divided by the total population
- Incidence = number of New cases within a certain time period divided by the total number of susceptible individuals in the population
In most cases, incidence and prevalence are directly proportional. When one goes up the other one goes up and vice versa. This intuitively makes sense. If you have more new cases of diabetes within a given year then you are likely to have a higher total number of people with diabetes at any particular point during that year. However, Prevalence and Incidence are not always directly proportional and test makers like to focus on these situations. Most of these situations include a change in the duration of the disease. Duration of a disease is the time from when a patient is diagnosed until they are cured or die. When duration is held constant, prevalence and incidence are directly proportional. The relationship between prevalence, incidence and duration of disease can illustrated with a simple formula.
Prevalence = Incidence * Duration
This relationship makes sense if you think about extreme examples. Consider a situation where there are 100 new cases of a disease per year but the disease only lasts one day. Annual incidence will be higher than prevalence as at any particular moment there is likely only going to be at most 1 person with the disease. Now consider a disease that has 100 new cases a year and the disease lasts for 40 years. The point prevalence is going to be higher than annual incidence, as at any given point you have the 100 or so newly diagnosed patients from this year plus people that have been diagnosed over the last 40 years that do not contribute to incidence.
Another way to think about the relationship between incidence and prevalence is the “Sink Metaphor.” The water coming into the sink from the faucet represents incidence and the newly diagnosed patients. The level of water building up in the sink is prevalence (the total number of patients at that moment). The drain of the sink represents patients either being cured or dying. If patients are cured quickly or are dying quickly then the level of the sink won’t be very high because the drain is really big. If there are very few patients dying or being cured that is like the drain getting clogged up and the sink backing up.
Case-Fatality Rate is a proportion of the people with a particular disease that die as a result of that disease. As the name implies, it compares the number of cases to the number of fatalities related to that disease. So if 20 people have a particular cancer and that cancer is fatal in 5 of them the case fatality rate is 5/20.
Picture Used: “Sink” by Last-Dino available at http://openclipart.org/detail/183124/sink-by-last-dino-183124as part of Public Domain