...................Trisomy 21............ Normal karyotype

Positive test .....100......................50

Negative test.....100....................250

An experimental diagnostic test is developed to noninvasively detect the presence of trisomy 21, Down's syndrome. The test is administered to a group of 500 women considered to be at risk for a Down's fetus based on blood tests. The results of this test are shown above. What is the sensitivity of this new test?

A. 40%

B. 50%

C. 67%

D. 71%

E. 83%

B......................

 

Explanation:

The correct answer is B. Sensitivity is defined as the ability of a test to detect the presence of a disease in those
who truly have the disease. It is calculated as the number of people with a disease who test positive (true
positive) divided by the total number of people who have the disease (true positive + false negative). In this
case, sensitivity equals the number of babies born with trisomy 21 whose mothers tested positive (100) divided
by the total number of babies born with trisomy 21. This yields 100/200 = 50% (not a very sensitive test).

40% (choice A) corresponds to the prevalence of the disease in the tested population, which in this case equals
the total number of babies with Down's syndrome (true positive + false negative = 100 + 100= 200) divided by
the total number of people tested (500). This yields 200/500= 40%.

67% (choice C) corresponds to the positive predictive value of the test, which equals the number of babies with
Down's whose mothers test positive (true positives = 100) divided by the total number of mothers testing positive
(true positive + false positive = 150). This yields 100/150 = 67%.

71% (choice D) corresponds to the negative predictive value of the test, which equals the number of normal
babies whose mothers tested negative (250) divided by the total number of people testing negative (350). This
yields 250/350 = 71%.

83% (choice E) corresponds to the specificity of the test, which equals the number of babies without Down's
whose mothers tested negative (250) divided by the total number of babies without Down's (300). This yields
250/300 = 83%.

An easy way to remember these concepts is:

Sensitivity = true positives/all diseased

Specificity = true negatives/all normal

PPV = true positives/all positives

NPV = true negatives/all negatives

Prevalence = all diseased/total population