Question

Upon returning from a 4-month study abroad experience in the Philippines, you begin to experience frequent pain and discomfort in your stomach and lower esophagus. Your doctor suggests this could be due to simple readjustment to western-style food, or could be due to an infectious agent – it turns out that roughly 19% of travelers who visit Southeast Asia for periods of 3 months or longer are exposed to and infected with Helicobacter pylori, a bacterium that is associated with peptic ulcer disease. Your doctor tests for this bacterium using a diagnostic that has 86% specificity and 94% sensitivity. Given this information, explain your reaction to the following two outcomes (use numbers to back up your statements and be sure to show your work to ensure partial credit):

Scenario 1: the test results are positive – how concerned are you that you are actually infected with this bacterium? (4 points)

Scenario 2: the test results are negative – are you confident that you are really uninfected? (4 points)

Answer 1

Scenario 1:

The test has 94% sensitivity. In other words, if a person is actually infected by H. pylori, 94 out of 100 times, the test will give positive results. This is fairly high sensitivity, and since the test results are positive, there are good chances that I am actually infected with this bacterium, especially given the fact that I have returned after a 4-month visit from an area, where only 3 months' visit suffices to cause infection in 19% of the individuals.

Scenario 2:

The diagnostic test has 86% specificity. In other words, if test shows negative results, only 86 out of 100 times the person is not infected with H. pylori. Another 14 (100-86) times, the test may give negative results despite H. pylori infection. This is not very high specificity, and I may still have the infection although the test is negative.