In this video we cover an overview of the following iterms in preparation for the RCGP AKT.,

Sensitivity and Specificity

Just because a test result is ‘positive’ or ‘negative’ doesn’t automatically make it so that the disease it is looking for is there or not. Take for example ‘d-dimer’ and ‘deep vein thrombosis’, it is commonly in practice if a patient has a positive d-dimer then this would require further investigation with an ultrasound. If the d-dimer comes back as negative then we are happy that it is very unlikely to have a DVT there and so often no further investigations are performed.

When considering sensitivity and specificity it is a formula based on whether two tests agree and to what level they agree. We shall call these tests A & B. Test A is the ‘screening’ test and is often cheap and easy to administer and process. Test B is normally more expensive, complex and time consuming and gives the most reliable result to whether a disease is present or not.

Let’s take COVID 19 as an example. Test A would be the home ‘lateral flow tests (LFTs) compared to Test B Polymerised Chain Reaction (PCR) tests. LFTs were cheap, easy and sent through the post to anyone who wanted them, I still have some in the back of my cupboard. PCR on the other hand, required booking an appointment, trained people to take the sample, then the sample is sent to a laboratory and processed.

You can see then the balance from the LFTs vs PCR is to see whether you can balance ease of the testing, costs vs this not being as accurate.

For many clinical situations, Test B is often been tested to the degree that they are considered to you ‘having the condition’ and considered to be one and the same.

In this video we go into detail on how to calculate the following based on these two tests:

1) Sensitivity

2) Specificity

3) Positive predicitive value

4) Negative predicitive value

5) Prevalence

Lastly we will touch on pre and post test probility.