*Concepts of sensitivity & specificity in Medicine* I’ve failed to understand these many times, the maths it was presented with made it even harder to understand. I imagine I might not be the only one who had trouble understanding these so I thought I’d make a thread explaining.

These are important concepts for assessing the significance of a test in a diagnostic work up, hence I’ll only explain my (practical?) way of understanding these and I’ll pay you the favour of omitting any maths.

Specificity means the same it means linguistically. When a positive test points to a SPECIFIC diagnosis it is said to be specific. In other words, a (positive) specific test allows us to AFFIRM a specific diagnosis. A test that is 100% specific is said to be pathognomonic.

Sensitivity. In common language, when we say for example someone is sensitive, we mean their emotions are easily triggered. Similarly a test that is sensitive is easily & OFTEN rendered positive by a said pathology. But it can also come out as positive because of OTHER diagnosis.

As such, it can’t be used to affirm a specific diagnosis, but it can allow us to exclude a diagnosis if it comes out as negative.

For example D-dimer is a sensitive test for PE. When there is a PE most times the D-dimer will be positive. So if D-dimer is neg. , we can exclude PE. But when it is pos. , this doesn’t necessarily mean the diagnosis is PE as other things such as DIC also give a positive D-dimer.

Ok. If you haven’t understood anything so far all you need to know is. A (positive) SPECIFIC test allows us to AFFIRM a SPECIFIC diagnosis. A (negative) SENSITIVE test allows us to EXCLUDE a diagnosis.

If you’ve understood this now, try to establish how useful a test would be in affirming/excluding something whenever you read about one and eventually the terms will become second nature to you. Reading this thread a few times may also help.

Hope this helps someone, somewhere. Feel free to correct any mistakes I might have made. #MedEd