 Welcome back to our MedSmarter question of the week where we're taking a smarter approach to preparing future physicians. Before we get started, if you'll take just a quick minute and click that like button and also subscribe and turn the bell on so that you'll be notified when we post new videos. Let's get right to that question and as always we start with the last sentence of the vignette before we read the rest of the question. Which of the following percentages indicates the specificity of this new test? A new nasal swab test was recently developed to detect antibodies to SARS-CoV-2 in order to diagnose the infection. 1,000 patients received the test and while 100 people had the infection, only 80 of them tested positive. Of the 900 people who did not have the infection, 800 tested negative and 100 tested positive. So we want to know the specificity of this test. Understanding sensitivity and specificity is very important. You need to be able to do these calculations so we're going to go through this one here today. Before we do that, take a minute, look at these answer choices, do the calculation yourself, come up with an answer and write it in the text box below. Alright, first thing we're going to do here is we're going to pull out our 2x2 chart. So we're going to come in here and draw our box and do this on the test. And we know that the top left is our true positive, then we have our false positive, false negative, and true negative. So true positive, that's going to be the patients that tested positive and had the disease or infection. So 80 are true positives. We also see, so 80 are true positives. We see that there were 100 people who had the infection. So if 80 tested positive, that means we had 20 that had a false negative. Then we had 900 people who did not have the infection, 800 tested negative, so that's our true negative. So 800 had a true negative and then 100 tested positive, but that was incorrectly tested positive, so that means they were a false positive. So now that we've got these determined, we need to pull out our calculation. So if you have a cheat sheet to help you with these calculations, you will know that the way to calculate specificity is going to be true negative divided by the sum of true negative plus false positive. So in this case, we can plug these numbers in. We have 800 over 800 plus our false positives of 100. That gives us 800 over 900 or 8 over 9. And you can do that calculation out there and you're going to come up with a number right around this, 89%. Therefore, my final answer to this question is 89% or D. And D is the correct answer. So like we said here, all you need to know in these instances is the calculation for specificity. Now, you obviously are going to need to know sensitivity, positive predictive value, negative predictive value, multiple other calculations. So for this particular question, we're needing specificity. And to get that, we're doing true negative over true negative plus false positive. So that tells us how specific these tests are to give us a true negative or a true positive case. Most important in this case is that high specificity gives us the opportunity for people that are healthy to test negative and know that they are truly negative and not undergo unnecessary treatments. So since you know that formula and you're able to break these out into their correct forms, we're talking about our true positives, our false positives, our false negative and our true negative. You can break all that out, you can plug these numbers in, you can get your answer and move right along with the questions. These are easy points to get and I hope this helps. If you found this material helpful for your studying, please like and consider subscribing to the channel. Also, share this video so that more people can benefit from it like you have.