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Incidence rates and recovery

Discussion in 'Diagnostic Guidelines' started by Graham, Aug 11, 2020.

  1. Graham

    Graham Senior Member (Voting Rights)

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    I have been puzzling about incidence rates, prevalence and recovery rates for some years now. Am I correct in assuming an incidence rate reflects the probability that someone will fall ill with the condition during that year, that prevalence indicates the number or proportion of people with the illness in the community, and that recovery rates reflect the probability that someone will recover from the condition during that year?

    If so I started to produce a spreadsheet looking at numbers in the UK. It is a very primitive one, but one I can modify to reflect reality a little more closely. But I am not confident of my mathematical analysis these days, so have attached it.

    I assumed a population of 64 million in the UK, a life expectancy of 80, no difference between males and females, no death rates, a uniform spread of people in each age group, and no recovery. I know these are daft simplifications, but it gets the layout going.

    The problem is that to reach a total of 250,000 people with ME in the UK, and assuming that anyone from 12 to 75 can be diagnosed with ME, I need a prevalence rate of 0.6%.

    A uniform prevalence rate is reasonable (I know people suggest there are peaks, but the PACE trial had a pretty uniform age distribution).

    Before I start to tweak things, am I on the right tracks?
     

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  2. Michiel Tack

    Michiel Tack Senior Member (Voting Rights)

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    To what if I may ask? What is the goal?

    It seems that you have calculated the prevalence rate needed to get to a population of 250.000 ME/CFS patients in the UK assuming only people aged 12 to 75 can be diagnosed with it.

    But the 250.000 figure in the UK is probably just a rough estimate based on US prevalence figures. I suspect someone quickly extrapolated the 0.42% figure from the Chicago study to the whole of the UK population (so including the youngest and oldest persons) a while ago. So I'm not sure why you need to reach the 250.000 figure.
     
  3. Amw66

    Amw66 Senior Member (Voting Rights)

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    I would guard against prevalence rates being uniformly distributed throughout the population. Given that we know certain age ranges have spikes, it may be that these cohorts have greater incidence. Discounting Crawley et al rates of c 2-2.5 % due to general lack of discrimination in diagnosis, International primer for paediatrics suggests 0.5 - 1%, which from memory seems to have come up in other research (can't remember where just now)

    The youngest diagnosis I could find on parents groups was 2(!); apart from the teenage spike years there is also a not insignificant number of kids from around age 9 that seem to be affected - perhaps when hormones start kicking off.

    There is also the confounding factor of recurrence, which dosn't seem to have much research at all.
     
  4. Graham

    Graham Senior Member (Voting Rights)

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    @Amw66 it is only a rough calculation: there is no point in refining it if my understanding is wrong, and to be honest, there isn't much value in tweaking it all. But different incidence rates with different cohorts will not have a major impact on the overall numbers. Starting at age 12 was just a random thought: my own son went down with it when he was eight.

    Recurrence implies recovery, and I am yet to be convinced that anyone recovers from ME. I know people tell me of people that they know have recovered, but out of the hundred (or many more) people that I know with ME, only two went through a stage that they thought was recovery, but turned out to be a patch of remission. I personally prefer the term remission – it reminds folk not to take things for granted should they be so fortunate.

    There isn't a goal. I'm just trying to square things in my mind. We seem to need an incidence rate of 0.6% to reach the 250,000 figure, the figure quoted by the charities: an incidence rate of 0.4% brings it down to 180,000: I've seen quotes of 0.2% which would mean 90,000. All that is assuming a trivial or zero recovery rate.

    My town has a population of 20,000 so an incidence rate of 0.4% would mean about 56 people having ME, which is possible, but a bit higher than I thought.

    You know I'm a geek: playing with numbers keeps me out of trouble.
     
  5. Amw66

    Amw66 Senior Member (Voting Rights)

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    yes, I would agree with remission v recovered- an area that needs some serious research too as there are numerous anecdotes of teenagers " recovering" only to become ill again in later life - it may be valuable to unpick what instigated recurrance - viral/ environmental/etc as it may hold clues for the onset too.
    As we know , recovered is a moving target depending on who is defining it.
     
  6. Michiel Tack

    Michiel Tack Senior Member (Voting Rights)

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    Do you mean a prevalence rate instead of incidence rate? I don't think incidence and recovery are relevant in determining how many ME/CFS patients there are at a certain time point.

    If you take 0.4% of 64 million, you get close to 250.000 so I suspect that's what someone did to get to the 250.000 figure (so without taking into account that the 0.4% only applies to the adult population).
     
  7. NelliePledge

    NelliePledge Moderator Staff Member

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    When I google uk population I get a figure of almost 68 million.
     
  8. Trish

    Trish Moderator Staff Member

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    If there were a uniform incidence of new cases in each age group each year, and nobody recovered, then there would be increasing numbers of sick people in each age group.

    For example, in the 30 year old age group, there would be people who got sick 20 years ago at age 10, 19 years ago at age 11 etc etc. as well as the new ones who got sick at 30.

    I'll have a go at doing some calculations, but I don't trust my brain to be working today.
     
  9. MeSci

    MeSci Senior Member (Voting Rights)

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    Does that allow for people who die?
     
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  10. Graham

    Graham Senior Member (Voting Rights)

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    That's where my brain starts to get confused. I think I have gone wrong, and need to insert a running total next to column B, and it is that total that represents the number of people infected: the number infected in each age group plus all the people in that age group who were infected at an earlier age.


    Not really, but capping the maximum age at 80 is a rough approximation. If I get my head around it correctly, then the next thing I could do is put proper numbers for each age group rather than assume they are all the same, and make some sort of allowance for the numbers of people with ME that die. But to be honest, that is probably more work than is appropriate for the sort of rough calculation I am doing.
     
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  11. Graham

    Graham Senior Member (Voting Rights)

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    I have just tried the running total, and that puts the numbers of patients in the millions. There's something fundamental here that I cannot get my head around, and it's frustrating. You need an incidence rate of 0.01% to get the overall numbers down to 166,000.
     
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  12. Trish

    Trish Moderator Staff Member

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    Here's my calculation so far:
    I've done it on a spreadsheet so I can change the figures easily but I can't work out how to embed a spreadsheet here.

    Population 68,000,000
    Age of death for everyone in the population 80
    Guestimate incidence of new cases/ year = 0.0001 = r
    Population at each age = 68,000,000 / 80 = 850,000
    New cases at each age = d = a = r x 850,000 = 85
    So there are 85 sick 1 year olds, 170 sick 2 year olds etc.
    Total number of patients at age n: An = a + (n - 1) d (nth term of an AP)
    So total number of sick 80 year olds is 85 + 79 x 85 = 6800
    They all drop dead, and the next year 80 x 85 = 6800 new cases arise spread across all ages
    Total number of sick people at any one time = formula for the sum of an AP
    Sn = n/2 [ 2a + (n - 1) d]
    = 80/2 [ 2 x 85 + (80 - 1) x 85]
    = 275,400

    Population prevalence = 275,400/68,000,000
    = 0.00405
    =0.4%

    Do tell me if I've made a mistake. Quite likely!
     
    Last edited: Aug 12, 2020
  13. Trish

    Trish Moderator Staff Member

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    I think you and I have done more or less the same calculation, @Graham, but I simplified it further and assumed new cases were evenly spread over the whole age range from 1 to 80. An incidence of new cases per year of 0.01% on my calculation gives a population prevalence of total cases, not just new ones, of 0.4%

    Now we need to refine the model by adding in death rates, recovery rates, and variable incidence of new cases in different age groups, such as much increased incidence among teens and 30-40's.
     
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  14. Peter Trewhitt

    Peter Trewhitt Senior Member (Voting Rights)

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    Given we have to rely on subjective reporting of symptoms at present, we can not conclusively distinguish between remission (an ongoing underlying condition that is largely asymptomatic) and recovery (no longer have the underlying biomedical condition).

    I was for a couple of years at a level of improvement that I would have at the time described as total recovery. I was conscious of none of the previous symptoms, but was being cautious about building back to my full premorbid activity levels. I was working half time, but took on the renovation of an eight bedroom main house with two associated holiday lets, so though cautious was still doing a awful lot.

    My relapse was instantaneous and severe following a very bad dose of seasonal flue over the millennium new year (my millennium bug). And no period of improvement since then has remotely approached something I could subjectively regard as recovery, just partial remission. Interestingly over time relapses have added to my symptom profile and the nature of my current disability could be seen as very different perhaps even qualitatively different to my initial disease.

    In retrospect I refer back to that period of being symptom free as remission rather than recovery, however my case could equally logically be described as a period of complete recovery with a second new case of ME triggered by a new infection, or as a period of remission with a relapse triggered by the new infection. Had I had say MS, brain scans might have provided a way of distinguishing between the two, but we do not yet have that option.

    In these situations I think it is important to take seriously people’s own account, whilst retaining a degree of skepticism. But ultimately until we have an agreed biomedical diagnostic tool, there will remain a degree of uncertainty.

    What we do lack is an accurate set of data: on how many people achieve some degree of recovery/remission, how often they are then totally symptom free or have intermittent symptoms, that allows them to return to their premorbid activity level; on how many people, who believed themselves to be in complete recovery or in a remission that allows a return to ‘normal’ life, have subsequent relapse; what sort of time spans this involves; and how many people have further relapses and remissions after this.

    All we can say is that an unknown percentage of people having been diagnosed as having CFS or ME under the current unsatisfactory system will experience some significant degree of remission, with some believing they have had a total recovery. Of these recoveries/remissions a further unknown percentage will relapse and we have no idea what the subsequent course of their condition will be.

    Unfortunately most of the current questionnaires do not measure adequately what we as patients would understand as ME, so to do a prospective longitudinal study, or even a retrospective study, require a better way of quantifying our self reported symptoms. Currently in terms of anecdotal evidence we in the ME world are likely to know more people who have relapsed than have continued recovered/in remission. Also in general those that promote their ‘recovery’ are already high profile or have something to sell, so may not be particularly representative.
     
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  15. Graham

    Graham Senior Member (Voting Rights)

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    I think it would be fair to say, looking at my calculation and Trish's, than any incidence rate per year above 0.01% would necessitate high recovery rates: ones that are easily visible. Refining the calculation will modify the numbers, but to a mathematician's eye, the figures are in the right ball park.
     
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