Question

RPR tests for syphilis have a sensitivity of 86% and a specificity of approximately 92%. Given a syphilis prevalence rate of 5 cases per 100,000 people.

Positive Predictive Value of 0.054% and Negative Predictive Value of 99.99%

1) What does this suggest about the usefulness of screening the general population? Please explain your answer (minimum 250 words)

Answer 1

The positive predictive value of 0.054% suggests that, if a person is tested positive, the probability that he/she will have syphilis is 0.00054. If a person is tested negative, according to the negative predictive value, the probability that he/she will have syphilis is 0.9999. This means that the test can predict with great accuracy if a person is not having syphilis, but the test predicting that a person is having syphilis is unreliable. In this scenario, the best way to use the test on general public is to exclude those one from further tests if one is tested negative by concluding that a negative test means that person is not having the disease. So, screening is to find out from the general population, a smaller group of people, who have higher probability of having the disease. Then in this smaller subset, detailed tests shall be conducted identify the presence of disease.

The reason why we are getting such results from the test is because of the fact that the prevalence rate of the disease in very low. In absolute numbers if we see, we get a large number of false positive results from the test, where as there is a very small number of results that are false negative.