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Quick Scroll SPM- calculation of PPV 11.20.05 (3 years ago) #1
If ELISA test has a sensitivity of 95% and a specificity of 95% find the positive predictive value of the test if it is used to screen a community with a prevalence if AIDS of 55.
a. 100
b. 75
c. 50
d. 25


How to solve the problem yaar...? icon_confused.gif
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Quick Scroll 11.20.05 (3 years ago) #2
Always make a 2X2 table first.. Then put in d values..

Usually they give prevalence per units, I presume its 5 per thousand. (At this point i dn't know wat 2 presume if nuthing is given in answer)

Now Actual AIDS cases in 1000=55

Actual people not having AIDS=945

Since sensitivity=95, True positive=95*55/100=52

Since specificity=95, True negatives=95*945/100=898

Hence False positives=Total healthy persons(945)-true negatives=945-898=47
******************
Now PPV= TP/(TP+FP)

So, PPV=52/(52+47)=52.5%
************************
Ans is 50%

Method is correct... But All values r approximate. Plus d fact what shud we take as d denominator in prevalence?????
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Quick Scroll 11.20.05 (3 years ago) #3
Sorry i meant i presume 55/1000.. icon_smile.gif icon_smile.gif
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Quick Scroll Re: SPM- calculation of PPV 11.21.05 (3 years ago) #4
every-b wrote:
If ELISA test has a sensitivity of 95% and a specificity of 95% find the positive predictive value of the test if it is used to screen a community with a prevalence if AIDS of 55.
a. 100
b. 75
c. 50
d. 25


How to solve the problem yaar...? icon_confused.gif



disease no disease

positive 95x55/1000 5x945/1000


negative 5x55/1000 95x945/1000

total 55 945


PPV = (95X55/1000) / [(95X55/1000)+(5X945/1000)]

= approx 50%

one thing I noticed is all these PPV Q s have an answr of 50%, when sens snd spec are in the range of 90%!!!

wats the scientific answer???
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Quick Scroll 11.21.05 (3 years ago) #5
dont read the previous post!!! i tried to post a 2x2 table!!!u but the allignment got bad....sorry...
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Quick Scroll 11.21.05 (3 years ago) #6
Yeah even i've noticed that gandalf.. But still its in our best interest to calculate without being biased... Isn't it?? icon_smile.gif icon_smile.gif icon_smile.gif

N dn't worry i've also given d same solution (not in 2x2 table format) cos i knew it wudn't appear properly.. icon_smile.gif icon_smile.gif
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Quick Scroll 11.21.05 (3 years ago) #7
!!!!!!!!
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Quick Scroll 11.22.05 (3 years ago) #8
thanks ashB and gandalf... i will refer the given ans and tell...
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Quick Scroll 11.25.05 (3 years ago) #9
Nice one, I was struggling on that since days !
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Quick Scroll 11.25.05 (3 years ago) #10
You may want to use this simple formula (Bayes theorem)
Predictive value of a positive test (PPV) = (Sensitivity) x (Prevalence) / (Sensitivity) x (Prevalence) + (1-Specificty)(1-Prevalence)


Predictive value of a negative test (NPV) = Specificity x (1 - Prevalence) / [Specificity x (1 - Prevalence)] + [(1 – Sensitivity) x Prevalence]


Hope it is legible

Dr Manish
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