one of the problems with the way we discuss Health interventions is that we see them in black or white something works or it doesn't something is good for you or it's bad for you things are rarely that simple though moreover there's good for you and good for you how do you know the difference that's the topic of this week's Healthcare [Music] triage let's say your chance of having a heart attack this year is 50% let's say I have a new drug that'll reduce that chance to 25% since we went from 50% to 25% I've effectively haveed your chance of having a heart attack that's great right let's say your chance of developing brain cancer this year is 0.5% let's say I have a new drug that'll reduce that chance to 0.25% again I've effectively haved your chance of brain cancer is that great in both cases I've haved your chance of disease this is known as a relative risk reduction you take the new risk and you divide it by the old risk in the first instance it's 25 over 50 in the second case it's .25 over5 in both cases it's 1/2 or 50% if the drugs are free and have no side effects then who cares you should take any kind of risk reduction but let's say the drugs are really expensive then are they worth it are those two things the same of course not in the first case you had a one and two chance of having a heart attack those are terrible odds you absolutely want to avoid that in the second case you start up with a 1 in2 200 chance of having brain cancer those are much better odds you're much more likely to take a chance there especially if the drug is expensive or dangerous relative risks are somewhat useless but those are the risks most often reported in news stories or trials that's because they almost always sound more impressive what we really should care about is absolute risk reduction to calculate that you take the old risk subtract the new risk and then divide by 100 let's work through these two scenarios I've already given you with respect to heart attacks we went from 50 50% to 25% that's 50 - 25 or 25 / 100 that's 0.25 so our absolute risk reduction is 25% with respect to the brain cancer example we went from .5% to 0.25% that's 0.5 minus 0.25 or 0.25 divided by 100 that's .0025 or 0.25% those numbers aren't even close the drug for heart attacks had an absolute risk reduction of 25% the drug for brain cancer had an absolute risk reduction of 0.25% one is a miracle the other one is much more debatable here's the thing though almost all of the therapies that we regard as awesome and necessary have shockingly low absolute risk reductions they've been sold to you in terms of relative risk reduction but that isn't telling you the whole story to the research last year the New England Journal of Medicine published a study touting the positive effects of the Mediterranean diet its conclusion was and I'm quoting among persons at high cardiovascular risk a Mediterranean diet supplemented with extra virgin olive oil or nuts reduce the incidence of major cardiovascular events how much of a benefit was actually seen though the absolute reduction of having a stroke heart attack or dying was 1.7% a study of high-risk smokers published also in the New England Journal of Medicine in 2011 said and I'm quoting again screening with the use of lowd do CT reduces mortality from lung cancer how much though the absolute risk reduction was 0.5% what about aspirin to prevent a first heart attack or stroke no brainer right I'm sure all of you have heard of that recommendation the evidence shows though that the absolute risk reduction is 0.06% you heard me but let's take absolute risk reduction a step forward that number can be used to calculate what we call the number needed to treat or nnt this refers to the number of people we need to to give a drug or therapy to in order for one person to receive the benefit I know that sounds a little odd but that's because you've been led to believe that therapies like benefits are black and white they work or they don't that's not how the world Works in reality things work on a spectrum some people receive a benefit some people don't and in the vast majority of cases way more people receive no benefit than people who do you calculate a number needed to treat our nnt by taking 100 and dividing it by the absolute risk reduction so going back to the heart attack drug which had an absolute risk reduction of 25% you take 100 divided by 25 you get four the nnt or number needed to treat is four that means that we have to give four people The Drug In order to have one receive the benefit in this case a prevented heart attack that means that three of the four people got no benefit at all none two of them would never have had a heart attack and one had a heart attack anyway even this miracle drug is is three times more likely to give you no benefit than to do what it's supposed to the brain cancer drug is much worse the absolute risk reduction was 0.25% so the nnt is 100 divided by 0.25 or 400 that means we need to treat 400 people with this drug in order to have one person receive the benefit of a prevented case of brain cancer 399 out of the 400 people who take this drug receive no benefit at all that's fine if the drug is cheap or if it has no side effects but almost no drugs have those characteristics so you have to ask yourself are you okay with being one of the 399 is a one in 400 chance worth it the Mediterranean diet that 1.7% absolute risk reduction translates into an nnt of 61 that means 61 people have to keep to this strict diet for 5 years for one of them to see a benefit the other 60 people saw no benefit at all is that worth it I'm not sure that's for each person to decide but I bet few people have been told that they're much much more likely to be doing this for nothing than for something lung cancer screening of high-risk smokers with CAT scans has an nnt of 217 to prevent one death that means the 26 people got scans and all the radiation with no benefit worth it and you need to treat 1,667 people with Aspirin for a whole year to prevent one first heart attack or stroke that means that 1,666 people got treated with the drug for a whole year with no benefit at all none some of you may think that any risk reduction is worth it maybe but you're not considering the harms that's the topic of next week's Healthcare triage