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# Sensitivity, specificity and predictive value

12. Table 1. Results of a screening test for diabetes, using a cutoff point of 160 mg.dl.

True Diagnosis
Test result Diabetic Not Diabetic Total
Positive 34 20 54
Negative 116 9,830 9,946
Total 150 9,850 10,000

The specificity of the screening test for diabetes is

a. 0.3%

b. 22.6%

c. 63.0%

d. 77.4%

e. 99.7%

13. The sensitivity of the screening test for diabetes is

a. 0.3%

b. 22.6%

c. 63.0%

d. 77.4%

e. 99.7%

14. The positive predictive value of the screening test for diabetes is

a. 0.3%

b. 22.6%

c. 63.0%

d. 77.4%

e. 99.7%

15. If the cutoff is set higher than 180 mg/dl, how would this affect the sensitivity and specificity?

a. no changes

b. sensitivity increase, specificity decrease

c. sensitivity decrease, specificity increase

d. both decrease

e. both increase

16. What will be the effect on false negatives (FN) and false positives (FP) of setting the cutoff higher than 180 mg/dl?

a. FP increase, FN unchanged
b. no changes

c. FP increase, FN decrease

d. FP decrease, FN increase

e. both decrease

17. When the screening cutoff point was lowered to a blood glucose value of 130 mg/dl, 164 persons now tested positive; 98 of these people were among the 9,850 persons judged by diagnostic tests not to have diabetes. Compute the sensitivity, specificity and predictive value of a positive test at this cutoff level in the next 3 questions Sensitivity is

a. 1%

b. 40%

c. 44%

d. 56%

e. 60%

f. 99%

18. Specificity is

a. 1%

b. 40%

c. 44%

d. 56%

e. 60%

f. 99%

19. Positive predictive value is

a. 1%

b. 40%

c. 44%

d. 56%

e. 60%

f. 99%

#### Solution Preview

12. (Points: 2)
Table 1. Results of a screening test for diabetes, using a cutoff point of 160 mg.dl.

True Diagnosis
Test result Diabetic Not Diabetic Total
Positive 34 20 54
Negative 116 9,830 9,946
Total 150 9,850 10,000

Condition is
Present Absent Total=
Test is Positive True Positive=34 False Positive=20 54
Negative False Negative=116 True Negative=9830 9946
Total= 150 9850 10000

The specificity of the screening test for diabetes is

a. 0.3%

b. 22.6%

c. 63.0%

d. 77.4%

e. 99.7%

Specificity = True Negatives/ (False Positive + True Negative)
= 9830 / (20 + 9830) = 99.7 %

13. (Points: 2)
The sensitivity of the screening test for diabetes is

a. 0.3%

b. 22.6%

c. 63.0%

d. 77.4%

e. 99.7%