Helical CT scanning may be a more sensitive screening test for lung cancer than
ID: 3179873 • Letter: H
Question
Helical CT scanning may be a more sensitive screening test for lung cancer than other tests such as conventional chest films or nuclear medicine studies. The Gold Standard for lung cancer diagnosis is finding cancer cells in bronchial washings. In a study, 200 individuals were screened for lung cancer using helical CT. In this population, 40 out of 43 who screened positive for lung cancer actually had the disease based on bronchial washings while 10 of 157 screened negative for the disease by helical CT actually had the disease based on bronchial washings.
Construct a 2 X 2 table showing Screening test vs. Gold Standard. Set up a 2 X 2 like Figure 11.4 in the text.
( 4 points)
Gold Standard (Bronchial Washing)
Present
Absent
Total
Test result CT Scan
Positive
a =
(True positives)
b =
(False Positives)
(a + b) =
+ Predictive Value
Negative
c =
(False Negatives)
d =
(True Negatives)
(c + d ) =
- Predictive Value
Total
(a + c) =
(b + d) =
Grand Total
(a + b + c + d)
Sensitivity
Specificity
Calculate the sensitivity ( 4 points)
Calculate the specificity ( 4 points)
10) Calculate the positive and negative predictive value ( 4 points)
11) Calculate the prevalence ( 4 points)
Calculate the accuracy ( 4 points)
Gold Standard (Bronchial Washing)
Present
Absent
Total
Test result CT Scan
Positive
a =
(True positives)
b =
(False Positives)
(a + b) =
+ Predictive Value
Negative
c =
(False Negatives)
d =
(True Negatives)
(c + d ) =
- Predictive Value
Total
(a + c) =
(b + d) =
Grand Total
(a + b + c + d)
Sensitivity
Specificity
Explanation / Answer
Gold standard (Bronchial Washing)
Present
Absent
Total
Test result CT Scan
Positive
a = true positive
40
b = false positives)
3
(a + b) = 43
+ Predictive value
93%
Negative
c = false negative
10
d = true negatives
147
(c+d) =
157
- Predictive value
93%
Total
(a + c) = 50
(b + d) = 150
Sensitivity (80%)
Specificity (98%)
Sensitivity = a/ b + c = 40/ 50 = 0.8 = 80%
Specificity = d/( d + b) = 147/150 = 0.98 = 98%
Positive predictive value = a/ (a + b) = 40/43 = 0.93 = 93%
Negative Predictive value = d/( c + d) = 147/ 157 = 0.93 = 93 %
Prevalence of disease = 50/200 = 0.25
Accuracy = (TN + TP)/(TN+TP+FN+FP) = (40 + 157) / 200 = 0.98 = 98%
Gold standard (Bronchial Washing)
Present
Absent
Total
Test result CT Scan
Positive
a = true positive
40
b = false positives)
3
(a + b) = 43
+ Predictive value
93%
Negative
c = false negative
10
d = true negatives
147
(c+d) =
157
- Predictive value
93%
Total
(a + c) = 50
(b + d) = 150
Sensitivity (80%)
Specificity (98%)
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