well welcome to another MedCram dot-com and board vitals question this one comes
from the USMLE step three question Bank and let's take a look here
so examine the following abstract and answer the question below let's take a
look at this abstract from a study background beta-blockers reduced
mortality in patients with chronic heart failure or systolic dysfunction and who
are on background treatment with diuretics and angiotensin converting
enzyme inhibitors this study seeks to determine whether or not carvedilol
reduces the risk of mortality compared to standard treatment with diuretics and
ACE inhibitors so the method in a multicenter double-blind and randomized
parallel group trial we assigned 1511 patients with chronic heart failure to
treatment with carvedilol target dose was 25 milligrams twice daily plus ACE
inhibitor and diuretic therapy and 1518 the standard treatment with combination
of ACE inhibitor and diuretic patients were required to have chronic heart
failure that's New York Heart Association classes two through four
previous admission for a cardiovascular reason and an ejection fraction of less
than 0.35 or 35% all patients also have been treated optimally with diuretics
and angiotensin converting enzyme inhibitors unless not tolerated primary
endpoints were all cause mortality and the composite endpoint of all cause
mortality or all caused admission analysis was conducted by intention to
treat let's go to the findings mean study duration was 58 months with a
standard deviation of 6 mean ejection fraction was 0.26 with a standard
deviation of zero point zero seven and mean age was 62 years with a standard
deviation of 11 all cause mortality was 34% and that was in 512 of the 1511
four carvedilol and 40% and that's in the 600 of the 1500 and 18 for the
control group the hazard ratio was zero point eight three with a 95% confidence
interval of zero point seven four to zero point nine three which was
statistically significant at a p-value you of 0.001 7 reduction of all cause
mortality was consistent across predefined subgroups incidents of side
effects and drug withdrawals did not differ substantially between the two
study groups so after all of that here's the question which of the following
represents the number of patients needed to treat to save one life with addition
of beta blockers and here are the choices so what this boils down to is
how do you calculate the number needed to treat so let's talk about that so the
number needed to treat is a very simple equation but before we get to the
equation let's talk about what the number needed to treat actually means so
what you're looking for is how many people do we need to give an
intervention to before one of them actually has the desired output how many
people do we need to take away cigarettes from to prevent one lung
cancer okay that's the kind of the issue that we're dealing with and you can see
here that the lower the number the lower the number is the more powerful that
intervention is the one I love to talk about because it's so obvious is
parachutes and people jumping out of planes how many people jumping out of
planes do we need to put a parachute on to save one life of course I mean a
parachute is extremely effective you could make the argument that there are
some people that have survived jumping out of a plane without a parachute and
of course there's certainly some people that have jumped out of planes with
parachutes that have not but by far and large if you give somebody a parachute
who's going to jump out of a plane it's extremely effective and the number
needed to treat is good to be very very close to one okay so that one is kind of
the lowest number that you could possibly get so how do we actually
calculate it this is the equation for it and this is the one that I want you to
remember it's simply the number needed to treat is the number one divided by
the absolute risk reduction and here's the key word absolute so in other words
if something goes from let's say a forty percent rate of something bad happening
to a twenty percent rate of something bad happen
okay 40 going to 20 what's the absolute risk reduction the absolute risk
reduction is 40 minus 20 the absolute risk reduction would be 20 percent or in
this case would be put as a point 2 right because 20 percent is point 2 what
we're not talking about here is saying look there was a 50% reduction in
mortality so we're not looking for that number 50 all right even though there is
a 50% reduction that's the relative risk reduction that's not what we're looking
for we're actually looking for the absolute value of the difference between
these two groups and so in this case 40 going to 20 the absolute risk reduction
is 20 okay so that's an example let's actually go in to the question stem and
get the numbers out of there so we know what we're doing so what I've done here
is I've highlighted in red the two numbers that we need everything
else in this question stem everything is superfluous we don't need any of it I
mean the background the methods and you should know that when you get down to
the bottom and they're talking here about the number of patients needed to
treat you know that you're looking at the risk with an intervention and the
risk without an intervention and so let's look at that the all-cause
mortality so this is the outcome was 34 percent when you had carve a doll and it
was 40 percent when you didn't have carve a loss so we went from 40 percent
down to 34 percent and so the way we would write that in the denominator is
0.4 0 minus 0.3 4 and as you can see here this is the one that it matches
that is the correct way of doing the denominator and of course the one over
as we talked about with the equation with the number needed to treat so why
are these other ones wrong well this one's got it backwards
point 3/4 minus 0.4 0 what's that going to give you that's going to give you a
the same absolute number in terms of the number needed to treat but it's going to
be a minus number right and so we don't treat - people here this
looks like it's closed but again they forgot to convert percent into decimal
points so 40% is actually 0.4 zero here it's the same issue except it's just
backwards again you're gonna get a negative number and here this number
here fits in a red herring which is the hazard ratio okay so let's just do it
for the fun of it and let's see what it shows in terms of a number so again it's
good to be one over the absolute risk reduction so in this case it's going to
be one divided by as we said 0.4 zero minus 0.3 4 which is equal to 1 divided
by 0.06 which is equal to sixteen point six seven now depends on how you want to
report this typically if you're going to report a number needed to treat the
question is how many patients do you have to treat and we don't treat
fractions of patients so in this case we would probably round up some people
might say the purists might say if that there's any left over you've got to
round up because you have to treat a whole patient so you've got to treat 17
patients in this case to save one life let's do our favorite example here or at
least my favorite example of one over the parachute test or the vertical
challenge as it said what's the mortality rate if somebody jumps out of
a plane without a parachute let's just say for argument purposes it's a hundred
percent and what are the chances of someone surviving if they jump out of a
parachute and they're well-trained and they know how to pull a ripcord and all
that kind of stuff they know how to actually use the intervention well it's
pretty close to zero right so what would be the number needed to treat well 1
minus 0 is 1 and 1 over 1 is 1 so that shows that this is a very very
effective treatment for the vertical challenge and that is number needed to
treat I would definitely know this it always pops up on tests thanks for
joining us