Prevalence
Lead Author(s): Jeff Martin, MD
Definition of Prevalence
Prevalence counts existing disease diagnoses, usually at a single point in time.
Difference Between Prevalence and Incidence
The difference between incidence and prevalence is a fundamental distinction in epidemiology.
Prevalent cases of a disease will over-represent those with longer duration or survival.
- This can potentially introduce significant bias into a study of a disease and its risk facotrs.
The amount of the difference between incidence and prevalence is related to the time period during which incidence is measured and the average length of duration of the disease condition.
- In some instances prevalence may look similar to incidence and in others the two measures will be very different.
The concepts are not difficult to grasp but there are some subtleties in implementing them as diseases with gradual onset can be diagnosed at varying points in their development, cancer being the most common example.
Both incidence and prevalence can be affected by changes in methods of diagnosis and the ability to identify disease at earlier stages.
Prevalent Sampling
A type of prevalent sampling is to sample only prevalent cases.
One of the advantages of doing a case-control study in a single hospital is the ready access to the cases, so in general it should be possible to pick up incident cases in the hospital setting.
- But even with the advantage of access, it may be difficult to include all the incident cases for diseases with poor survival, e.g. glioma patients.
- It is possible that using prevalent cases may not bias the findings but there is usually no way to determine that and the possible bias could go in either direction with respect to the study hypothesis.
Incidence or Prevalence?
Report: HIV/AIDS infection rates drop in Uganda
KAMPALA, Sept. 10 (Kyodo) - Infection rates of the HIV/AIDS epidemic among Ugandan men, women and children dropped to 6.1% at the end of 2000 from 6.8% a year earlier, an official report shows that the results were obtained after testing the blood of women attending clinics in 15 hospitals around the country.
The report says the average rate of infection for urban areas fell from 10.9% to 8.7%. In rural areas, the average was 4.2%, not much different from the 4.3% average a year earlier. The highest infection rate of 30% was last reported in western Uganda in 1992.
Discussion of Report
Use of the word rate should imply that incidence is being measured. Reports like that cited above are common. What is reported as an infection rate is not in fact incidence but prevalence.
It is not immediately clear that the figures of 6.1% and 6.8% are not incidence because it might be possible (although very unlikely) to have one-year incidence rates of HIV infection that high in Africa, but the last sentence of the report gives an infection rate of 30%, a figure so high that it can only be prevalence. For it to be incidence 30% of the population of women attending clinics in Uganda would have to have been newly infected in a one-year period. No HIV infection rate this high has been seen in one-year anywhere.
All of the figures are prevalence as they are the proportion of women who tested positive in successive years in 15 Ugandan hospital clinics. The proportion testing positive is prevalence as it does not take into account whether the women had all been tested the year before and whether the positives were only among those testing HIV negative the year before.
Since this is a news service report, it isn't clear whether the WHO and MCR of Britain used the language of infection rates or whether that was introduced by the reporting. But in any case you will see the same use of rate in some of the medical literature.