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#1
10-24-2012
 USMLE Forums Veteran Steps History: Not yet Posts: 234 Threads: 120 Thanked 11 Times in 10 Posts Reputation: 21
Calculating the Number Needed to Treat?

several pts with HTN whom you have treated for years have recently had strokes. you conduct a trial which shows that a partictuar anti htn lowers risk from 8 per 1000 pts to 6 per 1000 pts with relative risk reduction of 25%. wat is the number need to treat?

a. 4
b. 250
c. 500
d. 1000
e. 750

#2
10-24-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,216 Threads: 41 Thanked 1,327 Times in 711 Posts Reputation: 1342

C) 500

NNT is 1/ARR or 1/(8/1000-6/1000)

1/(.008-.006)
1/.002 = 500

You just have to divide 8 /1000 and 6/1000 substract and that result divided by 1 and you get the answer

If they give you Absolute risk reduction in the question stem then is even easier because you just divide 1/ ARR
 The above post was thanked by: eriqaly (08-17-2015), Novobiocin (10-24-2012)
#3
10-24-2012
 USMLE Forums Addict Steps History: Step 1 Only Posts: 136 Threads: 18 Thanked 84 Times in 37 Posts Reputation: 94

xpaezx has correctly mentioned ...

the other Technic is that,,,

To save 2 more people(with anti htn drug; 8-6=2), I need to treat 1000 patients; so how many patients will i have to treat to save 1 ?????

its simple in this question: 500..(1 * 1000 divided by 2)

in other questions,,yu can put it same way..

for eg,,,10 out of 1000 patient survive without Rx in certain disease, and 30 out of 500 survive with Rx with drug X..calculate the number needed to treat..

In this case, first we will have to equalize the number in both groups as number of patients in both the groups differ(500 & 1000)..we can consider that 60 out 1000 survive in Rx group (instead of 30 out of 500)....

Now, to save 50 more patients(60-10=50) , we need to treat 1000...so to save 1 patient we need to treat how many???

its 1*1000 divided by 50 = 20 patients...

It may seem confusing, but once this concept is understood, its really a faster way to calculate number needed to treat questions; without calculating Relative risk and absolute risk reduction...
 The above post was thanked by: eriqaly (08-17-2015), Imgfrmhalifax (10-24-2012), luschka (02-19-2013), Novobiocin (10-24-2012), Syndrome X (10-24-2012), vaibhavmanu (10-24-2012)

#4
10-24-2012
 USMLE Forums Guru Steps History: 1+CK+CS+3 Posts: 385 Threads: 6 Thanked 205 Times in 154 Posts Reputation: 215

Thanks buddy. That was a nice way to deal with this problem!
#5
10-24-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,867 Threads: 149 Thanked 2,106 Times in 1,049 Posts Reputation: 2126

Great explanations.

How do you calculate NNH ?
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#6
10-24-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,216 Threads: 41 Thanked 1,327 Times in 711 Posts Reputation: 1342

Quote:
 Originally Posted by Novobiocin Great explanations. How do you calculate NNH ?

I would like to assume is the same? but the question must ask that instead of NNT? and must imply in the vignette how many people were harmed and the like

 Tags Biostatistics-Epidemiology, Step-2-Questions

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