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Background reading for medical students Year 1 Semester 1 for PPSD session on 22nd August 2007 by Professor Omar Hasan Kasule, Sr.
DEFINITION OF PREVALENCE
Prevalence is a static concept that is a measure of state. It is a still-picture of the disease situation at a given point in time. Whereas incidence relates to events, prevalence relates to disease states at a point in time. The prevalence number is the number of cases of disease existing at the particular point in time. The prevalence proportion = # cases of illness at a particular time / # of individuals in the population at the same time. Prevalence proportion is also called prevalence rate or point prevalence. Prevalence is not a rate but a proportion; however the term prevalence rate has become so popular in medical literature that it will take long for this error to be corrected. Prevalence is measured in cross-sectional studies. Only one observation at one point in time is needed in the determination of prevalence.
PREVALENCE AS A PROPORTION
A proportion is the number of events expressed as a fraction of the total population at risk without a time dimension. The formula of a proportion is a/(a+b) and the numerator is part of the denominator. Like rates, proportions can be crude, specific, and standard. The variance of a sample proportion can be computed as {p(1-p)/n} if N is large in relation to n. Sample proportion can alternatively be computed as [{p(1-p)/n}1/2 {N-n}] / [N-1] If N is small in relation to n.
TYPES OF PREVALENCE MEASURES
Three types of prevalence are described in epidemiological literature: point and period prevalence. Point prevalence is a theoretical concept that assumes ability to count cases of illness at an infinitesimal short period of time. Period prevalence refers to counting the number of illnesses over a practically reasonable length of time. This must not be so long that there is a change in the status quo by death of cases or incidence of new ones. Period prevalence is more stable and therefore more useful than point prevalence.
RELATION OF PREVALENCE TO INCIDENCE
There is a relation among incidence, prevalence and duration. Prevalence proportion = incidence rate x average duration of disease. Change in prevalence is due to: (a) Change in incidence (b) Change in duration: due to dearth or recovery (c) Both change in IR and duration.
USES OF PREVALENCE
Prevalence is useful for administrative purposes. It is rarely used for etiological studies except for conditions in which incidence is difficult to measure such as congenital malformations, non lethal degenerative diseases, and sero-conversion. Prevalence is not good for etiological studies for the following reasons: (a) It can not distinguish the contribution of incidence from that of disease duration (b) The time sequence is not obvious; disease and exposure are studied at the same time.