2. Sensitivity of a test is 0.8 while the specificity is 0.6. The prevalence is

Question

2. Sensitivity of a test is 0.8 while the specificity is 0.6. The prevalence is reported to be 0.1. With your knowledge of 2 x 2 table, fill in the table and calculate positive and negative predictive values. Hint: Start by computing the total number of patients with and without disease. Then apply the sensitivity and specificity to those values to compute the gray areas of the table. Remember that prevalence is the proportion of patients who have the disease in the population in question. Disease No disease Test positive Test negative 1000 Use prevalence to calculate the number of diseased patients. With the sensitivity value, what is a. (i) the number of true positives? (0.5 points) (ii) the number of false negatives? (0.5 points) b. Use specificity and the number total number of non-diseased patients to calculate (i) true negatives (0.5 points) (ii) false positive? (0.5 points) Predictive value positive: Predictive value negative: (05 points) (0.5 points) =

Explanation / Answer

A) (i) Number of true positives = 0.1x0.8x1000= 80

(ii) Number of false negatives = 0.1x0.2x1000 = 20

B) True negatives = 0.9x0.6x1000 = 540

False positive = 0.9x0.4x1000 = 360

Positive predictive value = 80/440 = 0.1818

Negative predictive value = 540/560 = 0.9643

Disease No disease Test Positive 80 360 440 Test Negative 20 540 560 100 900 1000

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