Incidence and Prevalence – Everything you need to know
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hey everyone this video is about incidents and prevalence its absolutely crucial to understand what incidents and prevalence are and what the difference between the two is incidents is a measure of disease risk whereas prevalence measures disease burden incidence is defined as the number of new cases of a disease during a specific time period divided by the number of persons at risk for the disease during that same time period the resulting proportion can be multiplied by 1000 in order to get the number of new cases per 1000 population or by 100 in order to get the number of new cases per 100 population thats up to the beholder lets take the example of a fictitious population of ten women free from disease four of these women develop uterine cancer during a given year so the incidence is four divided by ten times 1000 equaling an incidence rate of 400 cases per 1000 population per year whats critical for incidents is that every person in the denominator must have the potential to become one of the people in the numerator in fact these four people are part of the denominator they transitioned from disease free to diseased incidents is a measure of events and therefore a measure of risk so we said that everyone in the denominator must have the potential to become a member of the numerator or the disease group lets come back to our study of uterine cancer if three women of our fictitious population had their uterus is removed if they had undergone hysterectomy in
other words they would no longer have the potential to become part of the numerator so instead of 4/10 it would now be 4/7 times 1,000 equals the new fictitious incidence rate of 570 per 1000 now what about prevalence prevalence is defined as the number of affected persons in the population divided by the number of all persons in that population at a specific point in time so in this fictitious example it would be 3/10 times 1,000 which would mean that we have a prevalence of 300 per 1,000 population and whats the difference between incidence and prevalence lets take a group of five people who develop tuberculosis over a period of six months one develops the disease in January two in February one in April and one in May so ignoring the denominator for now the incident cases in February for example would be – or in other words – would be the numerator of the incidence rate for that month and would be one for the months of January April and May now prevalence also takes into account the duration of the disease lets say case number one is cured in May case number two is cured in April case number three in March case four and five during the second half of the year now lets look at the prevalence of tuberculosis in these months lets also just focus on the numerator or the prevalent cases during each month we have one case in January three cases in February three and March three and April three in
May and two in June so prevalence is determined by how many people join the prevalence pool or the incidence and how many leave the prevalence pool through death or cure let us depict that with a vessel full of pebbles the vessel is filled with pebbles or new cases by the incidence rate and its emptied by death or cure this means that if death goes up then more people will leave the vessel and if death goes down then more people will stay in the vessel and the same thing applies for cure if more people are cured more pebbles or people will leave the vessel and if cure rate goes down then more pebbles or people will stay in the vessel lets have a closer look lets say this is our baseline prevalence now what happens to prevalence if incidence goes up while death and cure stay the same it has to go up right what happens if incidence stays the same but death goes down while curate stays the same it will go up since fewer pebbles will leave the vessel what if death goes down and curate goes up well as you probably guess correctly lens will decrease since more pebbles will leave the vessel so prevalence is a measure of disease burden as such it is a great tool for planning the allocation of Health Services so prevalence is incidence times duration and duration is determined by the death rate or the cure rate if the death rate or the cure rate go up then the
disease duration will be shortened if death rate and curate go down disease duration will be prolonged at most points in time prevalence will be lower with shorter duration and higher with longer duration of disease now lets use another fictitious example to clarify this point lets use an example of coronary heart disease in two different populations one is the population of Hollywood the other one is the population of the Bronx our fictitious Hollywood has a prevalence of 50 cases per 1000 population the Bronx on the other hand has a prevalence of 10 per 1000 so to the person whos not familiar with the fact that prevalence is not a measure of risk but rather a measure of the burden of the disease it looks like folks from Hollywood are at an increased risk of CHD but that conclusion is not true at all here lets look at the incidence rates duration of disease in both locations incidence is 5 per 1000 per year in both locations but duration of disease is 10 years in Hollywood due to better care whereas in the Bronx people die from CHD after 2 years so in fact the higher prevalence in Hollywood is just an expression of better care of an incurable disease you see we said prevalence is equal to incidence times duration so 5 times 10 is 50 and 5 times 2 is 10 so I hope the difference between incidence and prevalence is now crystal clear now I would love to hear your thoughts in the comment section below
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