Point Prevalence
Lead Author(s): Jeff Martin, MD
Definition of Point Prevalence
Point prevalence is the number of persons with disease in a time interval (eg, one year) divided by number of persons in the population; that is, prevalence at the beginning of an interval plus any incident cases.
The distinction between point prevalence and period prevalence is often not made because most prevalence estimates that you will encounter in the medical literature are point prevalence.
Example of Point Prevalence
The National Health and Nutrition Examination Survey (NHANES) III is a probability sample of all U.S. residents carried out from 1988 to 1994.
NHANES III was carried out over 6 years between 1988 and 1994. We are looking at the point prevalence of antibodies to the human immodeficiency virus (HIV) in these study patients. Each person’s blood was drawn at a single point in time. Of course, testing all of those blood samples together as if they were all drawn at a single point in time does make an assumption that the point prevalence wasn’t changing during the time it took to carry out the entire NHANES III study. This certainly would have been a poor assumption had NHANES III taken place in the early 1980’s, and it may also be questionable for the 1988 to 1994 time period.
The concept illustrated is that point prevalence is not a function of the length of time it takes to conduct the study.
- It is a function of the time period represented by the measurement.
- So a single blood sample represents one point in time.
- Although it may take a while to gather all of the samples, most prevalence studies don’t take 6 years, and the assumption of not much change in the prevalence being measured during the period of carrying out the study is usually reasonable.
HIV Prevalence in NHANES Population
McQuillan et al 1997 reported that:
Estimated national prevalence of 461,000 individuals or 0.18% of the population was HIV-infected.
This particular estimate of HIV prevalence was undoubtedly too low, as the authors acknowledged, because NHANES does not get a good sample of some of the groups at high risk for HIV infection, especially injecting drug users and probably gay men.
- They made some calculations and adjusted this estimate upward by about 200,000 persons.
References
McQuillan, G. M., Khare, M., Karon, J. M., Schable, C. A., & Vlahov, D. (1997). Update on the seroepidemiology of human immunodeficiency virus in the United States household population: NHANES III, 1988-1994. J Acquir Immune Defic Syndr Hum Retrovirol, 14(4), 355-360.