Question

What different types of inferences can be made from incidence and prevalence data?

Answer 1

Prevalence is the actual number of cases alive, with the disease either during a period of time (period prevalence) or at a particular date in time (point prevalence) but the prevalence will be influenced by both the rate at which new cases are occurring and the average duration of the disease.Incidence is the rate of new (or newly diagnosed) cases of the disease. It is generally reported as the number of new cases occurring within a period of time (e.g., per month, per year). It becomes more meaningful when the incidence rate is reported as a fraction of the population at risk of developing the disease. Average duration of disease is also important, because the only way you can stop being a prevalent case is to be cured or to move out of the population or die. e.g.few years ago average duration of lung cancer was about six months. Therapy was ineffective and almost all lung cancer cases died. From the time of diagnosis, the average survival was only about six months. So, the prevalence of lung cancer was fairly low but in contrast, diabetes has a long average duration, since it can't be cured, but it can be controlled with medications, so the average duration of diabetes is long, and the prevalence is fairly high.

The relationship between incidence and prevalence depends greatly on the natural history of the disease state being reported. For example an influenza epidemic, the incidence may be high but not contribute to much growth of prevalence because of the high, spontaneous rate of disease resolution. In the case of a disease that has a low (or zero) cure rate, but where maintenance treatment permits sustained survival, then incidence contributes to continuous growth of prevalence. In such cases, the limitation on prevalence growth is the mortality which occurs in the population. Obviously, prevalence will continue to grow until mortality equals or exceeds the incidence rate.

In steady state

prevalence is fairly constant and incidence and outflow [cure and death] are about equal), then the relationship among these three parameters can be described mathematically as:

P/(1-P) = IR x Avg. Duration,

where P= proportion of the population with the disease

(1-P) is the proportion without it,

IR is the incidence rate, and Avg. Duration is the average time that people have the disease (from diagnosis until they are either cured or die). If the frequency of disease is rare (i.e., <10% of the population has it), then the relationship can be expressed as follow:

Prevalence = (Incidence Rate) x (Average Duration of Disease)

This relationship can also be used to calculate the average duration of disease under steady state circumstances.

   Average Duration = (Prevalence) / (Incidence)

Example: Suppose the incidence rate of lung cancer is 60 new cancers per 100,000 P-Y, and the prevalence is 30 per 100,00 population, then

Average Duration of Disease = (30/10,000 persons / 60/100,000 person-years = 0.5 yrs A Conclusion can be made that : Individuals with lung cancer survived an average of 6 months from the time of diagnosis to death.

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