Sensitivity

Sensitivity (Sn) refers to the percentage of patients with a disease who test positive for the disease. Therefore, a sensitive test will correctly detect patients who have the disease. In other words, the sensitivity of a test refers to the probability that a true positive result will occur when used on a person with disease detected by a reference or "gold standard". True positive (TP) means a person tests positive and has the disease. A test with poor sensitivity will miss many patients with the condition. Therefore, patients will be incorrectly informed that they do not have a disease. This test result is known as false negative (FN). A test that is very sensitive should correctly identify most people who have the disease and a negative result can rule out the disease. We have a test we want to evaluate so we select 200 people, 100 have the disease and 100 do not. So, let’s see the test results of those who have the disease.

A way of remembering the concept of sensitivity is to remember the acronym “SnNOut”. A test with a high Sensitivity and a Negative result rules OUT the disease being present in that person. Thus, a negative test result in a highly sensitive test rules out the diagnosis. (Sackett, Strauss, Richardson, Rosenberg, Haynes, 2000). Sensitivity is calculated by taking the true positive rate and dividing it by the sum of the true positive + false negative [TP/(TP + FN)]. Screening tests are ordered to detect disease before it becomes symptomatic. If a screening test is positive, the patient must undergo further testing. Examples of screening tests include mammograms in women over 40 years of age and screening for inborn errors of metabolism in the newborn. Screening tests should be ordered if (1) the test will detect the condition earlier than it would have been detected without screening, and (2) if early treatment will make a significant difference in the outcome. A screening test should not be ordered if detecting the condition early does not improve health outcomes. Highly sensitive laboratory tests are used in screening or to rule out disease.