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#1
04-19-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 285 Threads: 32 Thanked 86 Times in 71 Posts Reputation: 96
sequential questions

An imaginary study involving 650 people was conducted to assess the performance of the stool antigen assay in detecting a helicobacter pylori infection. After collecting all the test results, it was found that 250 people tested positive and 400 people tested negative. If the positive and negative predictive value were 80% and 75%, respectively, then what proportion of people with the infection were identified by the test?

(A) 300/650
(B) 200/250
(C) 200/300
(D) 200/650

How would the sensitivity of the test change if the number of positive tests was higher?

(A) It would increase
(B) It depends on the prevalence
(C) Not enough information is provided
(D) It would decrease

What would the sensitivity of the test be if 10 people with the condition did not test negative?

(A) 200/310
(B) 210/300
(C) 210/310
(D) 200/300

#2
04-19-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 285 Threads: 32 Thanked 86 Times in 71 Posts Reputation: 96

Sample Table

How To Fill The Sample Table

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#3
04-20-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 285 Threads: 32 Thanked 86 Times in 71 Posts Reputation: 96

Answer to the first question is C

The question is asking for the true positive rate, which is the sensitivity of the test.

(A) 300/650 is the prevalence of the infection
(B) 200/250 is the negative predictive value
(C) 200/300 is the sensitivity
(D) 200/650 is the proportion of true positives in the study
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