USMLE Forums banner

Two False Positive/Negative Questions

3K views 6 replies 6 participants last post by  slime66 
#1 · (Edited)
1) A city with 1000 citizens has a 10% prevalence of disease x ..
Screening shows sensitivity 80% and specificity 70%
How many people without disease X will be falsely diagnosed positive by this screening test..
1) 20
2) 30
3) 270
4) 300


2) A city with 1000 citizens has a 10% prevalence of disease x ..
Screening shows sensitivity 80% and specificity 70%
How many people with disease X will be missed by this screening??
1) 20
2) 30
3) 270
4) 300
 
#5 ·
Explanations, please

:confused:
1) A city with 1000 citizens has a 10% prevalence of disease x ..
Screening shows sensitivity 80% and specificity 70%
How many people without disease X will be falsely diagnosed positive by this screening test..
1) 20
2) 30
3) 270
4) 300

2) A city with 1000 citizens has a 10% prevalence of disease x ..
Screening shows sensitivity 80% and specificity 70%
How many people with disease X will be missed by this screening??
1) 20
2) 30
3) 270
4) 300
 
#6 ·
Explanation

Always factor in prevalence. In this case, prevalence is 10% of 1000 people i.e. Diseased = 100 Non Diseased = 900 (1000-100)

TP = Sensitivity * Diseased people
= 80/100 * 100
= 80

TN = Specificity * Non Diseased
= 70/100 * 900
= 630

Thus, FP = Non Diseased - TN ( Those who were incorrectly identified as having the disease)
= 900 - 630
= 270 (answer 1)

Plug these values into a 4*4 table,

TP ------ FP
FN-------TN

Total has to equal 1000 (total population measured). Hence, those incorrectly diagnosed as not having the disease/ those this test misses = 20 (answer 2)
 
#7 ·
Thx a million!!!

:) thx and i really appreciated your explanations.
.
Always factor in prevalence. In this case, prevalence is 10% of 1000 people i.e. Diseased = 100 Non Diseased = 900 (1000-100)

TP = Sensitivity * Diseased people
= 80/100 * 100
= 80

TN = Specificity * Non Diseased
= 70/100 * 900
= 630

Thus, FP = Non Diseased - TN ( Those who were incorrectly identified as having the disease)
= 900 - 630
= 270 (answer 1)

Plug these values into a 4*4 table,

TP ------ FP
FN-------TN

Total has to equal 1000 (total population measured). Hence, those incorrectly diagnosed as not having the disease/ those this test misses = 20 (answer 2)
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top