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#1
05-31-2010
 USMLE Forums Veteran Steps History: 1 + CK Posts: 234 Threads: 63 Thanked 137 Times in 68 Posts Reputation: 167
A physician is deciding.....

A physician is deciding whether to use a new test to screen for disease X in his practice. The prevalence of disease
X is 5%. The sensitivity of the test is 85%, and the specificity is 75%. In a population of 1000, how many patients
will have the diagnosis of disease X missed by this test?

A. 50
B. 42
C. 8
D. 4

#2
06-01-2010
 USMLE Forums Guru Steps History: --- Posts: 487 Threads: 69 Thanked 341 Times in 163 Posts Reputation: 361
C

is it C 8??
how many patients will have the diagnosis of disease X missed by this test?
simply they r asking about False negative....they hv given 5% prevalence rate...so out of 1000.... 50 will hv disease...means in 2*2 table we hv TP+FN=50
sensitivity=TP/TP+FN=85%
TP=85% * (TP+FN)
TP=85*50/100
TP=42
we hv TP+FN=50....so FN is 50-TP=8
#3
06-01-2010
 USMLE Forums Scout Steps History: 1 + CS Posts: 22 Threads: 0 Thanked 30 Times in 13 Posts Reputation: 50

C Is correct: 8

 Tags Biostatistics-Epidemiology, Step-1-Questions

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