a. A screening test for meningitis, which involves PCR amplification of a sequen

Question

a. A screening test for meningitis, which involves PCR amplification of a sequence from Neisseria meningococcus, has a sensitivity of 22.5% and a specificity of 99.7%. The prevalence of meningitis is 2% in town X and 1.5% in town Y. The proportion of all positive results which are false positives is...? A. Higher in town X than in town Y. B. Higher in town Y than in town X C. The same in town X and town Y. D. Equal to the sensitivity of the test. E. Equal to the specificity of the test. b. Consider the information presented in the previous question. Assuming there are 10 000 people in town Y, how many people will test positive for meningitis? C. A new rapid test, based on immunochromatography, was developed to screen strep throat infections. The results of this test were compared to performing Gram stains on swabbed cultures grown on selective media. The results obtained are presented below Disease 100 80 | | -100 | 720 What are the specificity and sensitivity of this new test? What is the prevalence of strep throat in the population tested?

Explanation / Answer

a) P(+test / - disease for X) = P(X)*P(+ test/-disease) = 0.225*0.02 = 0.0045

P(+test / - disease for Y) = P(Y)*P(+ test/-disease) = 0.225*0.015 = 0.0033

A) Higher in town X than in town Y

b) P(test + for meningitities) = 0.225*10000 = 2250

c) sensitivity = 100/200 = 0.5

specificity = 80/800 = 0.1

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