prevelance and incidance

[arda]

---

Members don't see this ad.

Many books say that incidence does not change by advancement of treatment.

But if my prevalence is 10 out of 100 and if i use effective treatment which will increase prevalence to lets say 12 out of 100.People at risk of developing the disease is decreased from 90 to 88.So increasing prevalence should decrease incidence ?

 

[SBR249]

Incidence is generally unchanged as it reflects a certain risk in the population. So let's say 10% of a population gets a disease every year and you start measuring with 100 healthy adults. After 1 year you'd have 10 sick patients and 90 healthy. Assuming this is a chronic illness or that people don't survive more than a year, one year after that, you'd have 0.1*90 = 9 more sick patients. So the incidence remains 10% because nothing has changed the factors that make the population at risk. Let's say the 10 people used to die within a year but now treatment makes life expectancy 4 years, that means prevalence will increase but the incidence will remain 10% as the risk to the healthy population is unchanged.

Edit:

I think I know how you are approaching this. Questions stems normally give incidence as the number of occurrences in the overall population, like 10 out of 100. But incidence is really calculated as a percentage, denoting a risk/probability. So if the the population went from 90 to 88, the incidence rate will no longer be 10.

Edit2: I should note that when FA talks about "incidence" they generally mean "incidence rate" which is a proportion like prevalence.