Question

A screening test applied to 1500 subjects identified 250 of them as positive. On further investigation, 110 of these were found to have the disease being tested for. In subsequent follow-up, 20 of the subjects who tested negative developed the disease within a reasonably short time. Calculate the sensitivity, specificity, and positive predictive value of the test. Explain in words what a test specificity of 75% means? Would you recommend using this screening test? Explain your answer.

Answer 1

	disease	No disease	Total
Positive	110	140	250
Negative	20	1230	1250
Total	130	1270	1500

Sensitivity is the probability that a test will indicate 'disease' among those with the disease

= (110 / 130) x 100 = 84.62 %

Specificity is the fraction of those without disease who will have a negative test result

= (1230 / 1270) x 100 = 96.85%

Positive predictive value is the chance that a person with a positive test truly has the disease

= (110 / 140) x 100 = 78.57 %

As, the Sensitivity, Specificity and Positive predictive value of the test are high and greater than 75%, we recommend using this screening test.

A test specificity of 75% means that 75% of those without disease will have a negative test result.