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#1
04-10-2013
 USMLE Forums Veteran Steps History: CS Only Posts: 220 Threads: 45 Thanked 45 Times in 33 Posts Reputation: 55
Number Needed to Treat versus Number Needed to Harm?

Ive looked for the equations for those.. And found then to be the same!!
=1/absolute risk reduction

So whats the difference?!?

#2
04-10-2013
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 378 Threads: 68 Thanked 207 Times in 101 Posts Reputation: 217

nnt=1/absolute risk reduction
nnh=1\attributable risk
 The above post was thanked by: Faith-USMLE (04-10-2013), neha_subh (04-11-2013)
#3
04-10-2013
 USMLE Forums Veteran Steps History: CS Only Posts: 220 Threads: 45 Thanked 45 Times in 33 Posts Reputation: 55

Quote:
 Originally Posted by younissmed nnt=1/absolute risk reduction nnh=1\attributable risk
Isnt attributable risk the same as absolute risk reduction?

#4
04-10-2013
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 55 Threads: 7 Thanked 19 Times in 12 Posts Reputation: 29
hope this helps

number needed to treat : number of people you need to give experimental treatment to prevent 1% from getting the disease of interest.
Attributable Risk= difference of treated and untreated groups(to medicine)
NNT= 1/AR

number needed to harm : number of people you need to expose to risk factor in order to cause 1% to acquire the disease of interest.
AR= difference of exposed and unexposed (to risk factors)
NNH= 1/AR
 The above post was thanked by: Faith-USMLE (04-10-2013), neha_subh (04-11-2013)
#5
04-11-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 116 Times in 77 Posts Reputation: 126
q: no.needed to treat

Quote:
 Originally Posted by med_ziva number needed to treat : number of people you need to give experimental treatment to prevent 1% from getting the disease of interest. Attributable Risk= difference of treated and untreated groups(to medicine) NNT= 1/AR number needed to harm : number of people you need to expose to risk factor in order to cause 1% to acquire the disease of interest. AR= difference of exposed and unexposed (to risk factors) NNH= 1/AR
realy thanks.

now time to solve the question acording to definition.

Q:
Eficacy of new lipid lowering drug marketed (fibrates),clinical control trial at 5yrs given below:

-------------------Treat:with fibrates----------treat:with placebo

MI--------------------20-------------------------------35
non MI---------------980-----------------------------965

number needed to treat????
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#6
04-11-2013
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in my view

Quote:
 Originally Posted by neha_subh realy thanks. now time to solve the question acording to definition. Q: Eficacy of new lipid lowering drug marketed (fibrates),clinical control trial at 5yrs given below: -------------------Treat:with fibrates----------treat:with placebo MI--------------------20-------------------------------35 non MI---------------980-----------------------------965 number needed to treat????

First to make a table:

Drugs MI
+ -
Fibrates 20 980
placebo 35 965

Absolute Risk Reduction = Event rate in group taking placebo- Event rate in
group taking fibrates ( i.e. number of people in
study to be treated by the drug to prevent MI)

= 35/35+965 - 20/20+980
= 35/1000 - 20/1000
= 1/200
NNT= 1/ ARR = 200
#7
04-11-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 116 Times in 77 Posts Reputation: 126

Quote:
 Originally Posted by med_ziva First to make a table: Drugs MI + - Fibrates 20 980 placebo 35 965 Absolute Risk Reduction = Event rate in group taking placebo- Event rate in group taking fibrates ( i.e. number of people in study to be treated by the drug to prevent MI) = 35/35+965 - 20/20+980 = 20/1000 - 35/1000 = 1/200 NNT= 1/ ARR = 200
i have one question here is that event rate with placebo-event rate group taking drug? how we can put that equation in this scenerio
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#8
04-11-2013
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Quote:
 Originally Posted by neha_subh i have one question here is that event rate with placebo-event rate group taking drug? how we can put that equation in this scenerio
here it simply means that : the event rate in placebo group is the number of the total people in placebo group who had MI by taking the placebo ,out of total number of people receiving placebo.
similarly, event rate in fibrates group is the no. of total people who took the fibrates but still had MI .
So if we substract these two, we can get the total number of people who are at risk of getting MI if they dont take the fibrates drug (or take placebo in this case).
This gives us the absolute risk reduction. And hence number of people we need to treat in order to prevent MI will be 1/ ARR
 The above post was thanked by: neha_subh (04-11-2013)
#9
04-11-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 116 Times in 77 Posts Reputation: 126

Quote:
 Originally Posted by med_ziva First to make a table: Drugs MI + - Fibrates 20 980 placebo 35 965 Absolute Risk Reduction = Event rate in group taking placebo- Event rate in group taking fibrates ( i.e. number of people in study to be treated by the drug to prevent MI) = 35/35+965 - 20/20+980 = 35/1000 - 20/1000 = 1/200 NNT= 1/ ARR = 200

event rate with placebo -- event rate with treatment:

35/1000 -- 20/1000

0.035 -- 0.02 = 0.015
now
1 / ARR
1/ 0.015 = 66.66%

Am i right ???
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#10
04-11-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 116 Times in 77 Posts Reputation: 126

for number needed to harm:

First to make a table:

---------------------- MI-----------------no MI------------

exposure---------- 35------------------965
no exposure--------20 ----------------- 985

now

1/ AR = 35/1000 -- 20 / 1000

1 / AR = 0.035 -- 0.02
1/AR = 1/ 0.015 = 66.66%

Am i right?
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Last edited by neha_subh; 04-11-2013 at 02:49 AM.
#11
04-11-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 116 Times in 77 Posts Reputation: 126

MED_ZIVA its uw concept:

event rate with placebo -- event rate with treatment:

35/1000 -- 20/1000

0.035 -- 0.02 = 0.015
now
1 / ARR
1/ 0.015 = 66.66%
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 The above post was thanked by: med_ziva (04-11-2013)

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