Calculation of Sensitivity and Specificity Urine hCG tests are commonly used to
ID: 141978 • Letter: C
Question
Calculation of Sensitivity and Specificity
Urine hCG tests are commonly used to determine if someone is present. Urine pregnancy tests qualitatively detect the presence of hCG in the urine. While manufacturers often state that the tests have 99.99% accuracy, they are typically referring to the accuracy of the test in a patient who has a markedly elevated urine hCG. The table below contains data based on a population of women who are only a few weeks into their pregnancy.
Pregnant?
Positive urine hCG
Negative urine hCG
Total
Yes
32
8
40
No
5
143
148
Total
37
151
188
Use the information in the table to answer the following questions:
1. Calculate the sensitivity and specificity of urine hCG testing in this patient population
Pregnant?
Positive urine hCG
Negative urine hCG
Total
Yes
32
8
40
No
5
143
148
Total
37
151
188
Explanation / Answer
True positive i.e Pregnant women showing positive urine hCG = 32
True negative i.e non pregnant women showing negative urine hCG = 143
False positive i.e non pregnant women showing positive urine hCG = 5
False negative i.e pregnant women showing negative urine hCG = 8
Sensitivity of the assay is that the probablitiy that the test will detect pregnancy among pregnant women
It is calculated as : number of true positives / (number of true positive + number of false negative ) x 100
sensitivity = 32 / (32 + 8) x 100 = 32/40 x 100 = 80%
Specificity is the probability that non-pregnant women will show a negative result
It is calculated as number of true negative / (number of true negative + number of false positive ) x 100
Specificity = 143 / (143 + 5) x 100 = 96.6%
Sensitivity is 80% and specificity is 96.6%
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