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#1
02-17-2012
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Prevalence and Positive Predictive Value

In a nationwide study, 10% of the screening tests performed on patients with disease X resulted in false-negative outcomes, while 25% of the tests falsely identiﬁed healthy patients as having the disease. To determine the probability that a patient who tests positive for disease X actually does have the disease, what additional information is necessary?
(A) No additional information is necessary
(B) The incidence of the disease must be known
(C) The prevalence of the disease must be known
(D) The sensitivity of the test must be known
(E) The speciﬁcity of the test must be known

#2
02-17-2012
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the prevelance of the disease must be known...
low prevelance low positive predicted value n high negative predicted value
 The above post was thanked by: pass7 (02-17-2012)
#3
02-17-2012
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C) prevalence should be known

#4
11-16-2013
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First mention whether the screening tests used are same, or different types of tests are used on different patients. If the test used is a same one, then the sensitivity and the specificity for that specific test are already given in the question itself. Sensitivity is 90% and specificity is 75%. As they are asking for a Positive predictive value, sensitivity and specificity are not enough to derive + predictive value, we need to also have prevalence.
#5
11-16-2013
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Quote:
 Originally Posted by tyagee In a nationwide study, 10% of the screening tests performed on patients with disease X resulted in false-negative outcomes, while 25% of the tests falsely identiﬁed healthy patients as having the disease. To determine the probability that a patient who tests positive for disease X actually does have the disease, what additional information is necessary? (A) No additional information is necessary (B) The incidence of the disease must be known (C) The prevalence of the disease must be known (D) The sensitivity of the test must be known (E) The speciﬁcity of the test must be known

#6
11-16-2013
 USMLE Forums Guru Steps History: Step 1 Only Posts: 342 Threads: 47 Thanked 149 Times in 86 Posts Reputation: 159

For me its sensitivity.. Answer D i dont understand why u ppl thinks its C? Explain. For me please i am confused
#7
11-16-2013
 USMLE Forums Veteran Steps History: Step 1 Only Posts: 283 Threads: 35 Thanked 206 Times in 115 Posts Reputation: 216

Quote:
 Originally Posted by DrAGA For me its sensitivity.. Answer D i dont understand why u ppl thinks its C? Explain. For me please i am confused

Simple! PPV = prevalence. NPV is opposite of prevalence.

Sensitivity and Specificity has nothing to do ppv or npv.

 Tags Biostatistics-Epidemiology, Step-2-Questions

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