From clinical point of view, it is very important to understand the concept of sensitivity and specificity of a test.
Sensitivity: Suppose 10 subjects were screened for a test. In reality all 10 did have the disease but test came positive only for 8 people. This test failed to catch 2 people who had the disease. Test showed false negative tests for those two persons. This test is only 80 % sensitive.
Sensitivity is calculated by the following formula:
True positives/True positives+False negative
An ideal test will have a high sensitivity meaning it will catch all people with a disease. If a test has a low sensitivity, it wouldn't be reliable because as a clinician based in false negative, you will tell your patient that he/she does not have the disease while in reality he/she is diseased. It is like letting loose a criminal.
Specificity: Suppose 10 persons were tested for a disease and all 10 people came positive for that disease. In reality, only 8 people had the disease. The two people who came positive do not have the disease in reality. They have some other condition that gives a false positive.
If a test has a low specificity, it is not relaiable because it will give false positive results.
The formula for specificity is:
True negative/true negative+False positive
An example of a test that is not very specific is VDRL and RPR (syphilis diagnostic tests). These tests show positive results for SLE (systemic lupus erythematosus).
Any comments are welcomed.
Good luck in your studies.