What's the harm in screening?

What's the harm in screening? That's a good question, let's look at it.

For this, we need to understand the concept of sensitivity and specificity.

They're confusing words, so I'll explain. Sensitivity is the measure of a test, expressed as a percentage, that shows how accurate it is at detecting a disease. So if I have disease A, and the sensitivity is 100%, then the test will definitely identify that I have disease A. If it is less than 100%, then there's a chance that it would miss the fact that I have disease A. This is called a false negative. Sensitivity of 60%, means that it will miss 40% of disease A cases.

Specificity is the measure of a test to only detect disease A. If the specificity is 100%, and I don't have disease A, then I can be 100% confident that that result is correct. However, if the specificity is less than 100%, then I might be told that I have disease A, when actually I don't. This is called a false positive. Specificity of 60% means that 40% of those tested will be incorrectly diagnosed with disease A.

So, here are the harms of screening. Being told that you don't have disease A when you actually do, and therefore might ignore symptoms of disease A thinking that you're all clear. Or, being told that you do have disease A, when you don't, resulting in treatment that you didn't need, the side effects of that treatment, and the worry and anxiety that goes along with it.

In reality no screening test has a sensitivity and/or specificity of 100%, so there are always false negatives, and false positives. 

What we can do is understand these risks, enabling us to make an informed choice as to whether we want to participate in the screening program. Fully armed with the knowledge that we could be underdiagnosed, or overdiagnosed. Undertreated, or overtreated.