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Odds of Incidence and Lifetime Risk

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Raymond.L

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Hello everyone. I’ve been here for sometime and seen quite much anxiety over ALS from people in this forum (including me). Actually the incidence of this disease is very low. It is only around 2 or 3 or 4 in 100000 persons every year. The highest odds I ever saw was 10 in 100000 persons annually. I only saw this number once from a web page which I couldn’t find now. But there is another medical term called lifetime risk.

I don’t know how this term comes from but it is really said that an individual’s lifetime risk of developing ALS is around 1 to 1000. Like the incidence this odds also drifts vastly in different sources as far as I’ve read from the lowest 1/2000 to the highest 1/300 (which is truly not low).:(

How come is this number? Is it really reasonable? I thought it might be the product of the annual incidence and the average lifetime in years of an individual. But this doesn’t make sense. When calculating the annual incidence the denominator (or sample) is different every year because every year there are many people added into and leaving the sample for all kinds of reasons, which can make the annual incidence for every year irrelevant from each other. Let’s say the flu affects 20% of all the population of a country every year. Does it mean that the odds of one in this country getting flu during five years is 100%? :confused:So it’s confusing me now. Could someone knowing this please give an answer?

PS. Apologies for the English but I tried hard. Hopefully you understand what I’m saying. Thank you so much.
 

Vincent

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The medical literature puts it at 1/50000. So you really have to be some sort of special to get this unless you have the familial type. It is also more common for some reason in veterans.
Vincent
 

lgelb

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Raymond, you are right; it's not calculated as either of your straw men. Think of annual incidence and personal lifetime probability as separate probabilities, although statistically there are some interdependencies.

Incidence in a population is the number of cases newly diagnosed in a year. But we know from epidemiology that if you're older, a present/past smoker, exposed to certain toxins, etc. your personal lifetime risk is greater than a teenager's for your time remaining. But to be precise about lifetime risk, we'd have to know how much time you have left. That's another estimate that should be based on personal risk factors, family history, etc.

And of course the population incidence as you pointed out relates to the risk factors across that entire population. So that relates to other disorders and risks, as if we were all dying of heart attacks at 55, many more of us would escape ALS than occurs today. Every time there are wars, epidemics, earthquakes, as well as garden-variety deaths, the cohort is affected. Etc.

So the flu example works the same way, you can't extrapolate personal risk wholly from population incidence, nor from point prevalence (how many people have something at a given time, like we think about 30K people in the US have ALS at this moment).

Does that make any more sense? You are one member of the population so you contribute to the 20% get flu or 2 in 100000 get ALS but whatever we see about the population as a whole doesn't necessarily apply to you in equal measure. You're the only you in that population.

Hope this helps.
--Laurie
 
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Raymond.L

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Hello, Vincent and Laurie. Thank you very much for responding. Your words are very helpful and really make sense.

Human beings have conquered so many diseases that used to pose serious threats on people, like pneumonia and smallpox. Really hope there will be an effective cure soon for this one too.
 
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