COVID-19 (SARS-CoV2) Pandemic: Test Positivity Ratio and Death-to-test Ratio

by Raywat Deonandan, PhD

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    00:01 So, how do we know for testing enough? In absence of a cure, or treatment, or a vaccine testing is our best way of controlling this.

    00:11 So, WHO suggested, that we want to see countries testing on a level of 10 negative tests to one positive test.

    00:19 In other words, 10% of your tests should come back positive at the most.

    00:26 If it's more than that you're not testing enough.

    00:29 That's because at that point of the epidemic, it is unlikely that the prevalence of COVID-19 in your community is over 10%.

    00:38 And if you're getting that number, it means that you are simply not sampling enough people.

    00:43 So, it's a call to expand your testing regimen.

    00:48 Beyond simply the most serious cases showing up and presenting themselves.

    00:53 So that brings up an indicator called the Test Positivity Ratio, TPR.

    00:57 That's the number of tests that are positive, divided by the number of tests that you've conducted.

    01:02 It's a useful indicator for measuring your response to the epidemic, but also the nature of the epidemic.

    01:11 It's two indicators and what it tells us if you are testing enough, and it also tells us how well the disease has penetrated your community? Because if you haven't changed your testing strategy or volume over the course of a few weeks, and the test positivity rate increases, it means that the disease have a said deeper foothold in your community than you think.

    01:39 On the other hand, if you start to test more, and the test positivity rates diminishes substantially, that means the disease is not as well penetrated as you would have previously thought.

    01:49 That's a good thing.

    01:51 So the test positivity ratio can be used strategically, to make decisions and to monitor the rate of change of the epidemic in your community.

    02:02 Here is a comparison of different countries test positivity ratios on April 20th of 2020.

    02:10 By the way, I am interchangeably using the words test positivity rate and test positivity ratio.

    02:15 The appropriate term is ratio.

    02:17 But epidemiologists have this annoying habit of this saying rate when we mean ratio.

    02:24 That's just the nature of the game, it seems.

    02:26 So, back in April, the UK had a test positivity ratio of 30%, which is way too high.

    02:34 It means that they were not testing enough.

    02:36 They did not have their testing infrastructure as advanced as it should be.

    02:41 You look at the country is doing this well, Austria, South Korea, Iceland, these are countries that had a grasp early on of the testing challenges.

    02:53 This curves show us how tests were being performed in the early days of the epidemic from the start of 2019, sorry, the start of 2020 to the end of 2020.

    03:11 Different regions of the world took different paths.

    03:14 Many countries struggled to get their testing infrastructure well established.

    03:21 There are different types of tests that we can conduct.

    03:24 The diagnostic tests are important.

    03:27 The molecular test is the most common one conducted.

    03:30 It is the gold standard.

    03:31 The PCR tests, it detects presence of genetic material of the virus.

    03:38 The number of genes targeted varies.

    03:41 And a lot of work has gone into making sure that this is a well calibrated test, even though it was not designed as a diagnostic test in general.

    03:51 But it is our best tool to detect the presence of SARS-CoV2 in a sample.

    03:56 Antigen tests detect specific proteins on the surface of the virus.

    04:00 These are less accurate than molecular tests, but they're faster, and they're cheaper, and how we use them matters.

    04:10 So, in general, epidemiology, how you apply different kinds of tests varies with the characteristics of those tests.

    04:19 Testing can be used for diagnostic purposes.

    04:24 Patient shows up to the hospital, and we need to test them to figure out, what disease they have, so we know how to treat them.

    04:30 Testing can be used for surveillance purposes.

    04:34 So, we do a sample of the population, and we test them to see, where and how many cases there are in the community? And we do that repeatedly over the course of many weeks.

    04:47 That surveillance testing.

    04:49 Or testing can be done on a screening basis.

    04:52 So, the screen testing, we need to have a secondary test that is more accurate, that we confirm the first test with.

    05:01 So, consider a breast exam, a mammogram.

    05:06 Mammograms aren't particularly accurate.

    05:09 They have very high false positive rates.

    05:11 That's okay, because we have the biopsy to confirm each positive test.

    05:17 But mammograms are faster and cheaper than biopsy, so we can do them at a wider scale and capture more cases that way.

    05:25 So, an antigen test for COVID-19.

    05:27 If it's not highly accurate, if it's rapid and cheap, it can be used as a screening test because we have the PCR test as the backup.

    05:37 So, we need to be more creative, and imaginative in how we use these tests.

    05:42 We also have the antibody or serologic tests.

    05:46 In these tests determine who has already been exposed to the virus in the past because it tests for antibodies.

    05:54 But complicated because some people do not have lingering antibodies, they wane very quickly.

    06:00 So you might have been exposed months ago or weeks ago, but are no longer testing positive on the serologic test.

    06:06 It's a useful test though, for things like surveillance, sero-surveillance, and allowing us to determine the infection fatality rate in the population.

    06:18 It cannot be used to diagnose an active infection because the infection might have been weeks or months ago.

    06:27 Different tests must be used in different ways to provide different pieces of information to allow us to manage the spread of this disease.

    06:38 So, this map shows us where most positive tests are around the world.

    06:45 The darker the red, the more dire the situation.

    06:50 And the poor and more densely populated countries seem to be the most affected, with some glaring exceptions, notably the United States, which is really struggling with managing this disease.

    07:07 How do we know for testing enough? Another thing to consider, is maybe compare the number of deaths to test.

    07:14 So, one of my colleagues, Dr. Ashleigh Tuite said back in April, "Having more tests per death suggests that you are finding more of the less-severe infections.

    07:24 It means that you probably have a better view of what's going on in terms of the amount of disease that's in your population." So comparing testing to deaths is also useful.

    07:35 Here we have the number of positive tests per deaths around the world.

    07:40 And this can be used to determine the health and vivacity of a testing regimen of a nation.

    About the Lecture

    The lecture COVID-19 (SARS-CoV2) Pandemic: Test Positivity Ratio and Death-to-test Ratio by Raywat Deonandan, PhD is from the course Pandemics.

    Included Quiz Questions

    1. Italy
    2. Japan
    3. China
    4. Canada
    5. Germany
    1. Molecular test (RT-PCR)
    2. Antigen test
    3. Antibody test
    4. MRI scan
    5. CT scan
    1. Taiwan
    2. Singapore
    3. South Korea
    4. China
    5. England
    1. 10 negative tests to 1 positive test
    2. 10 negative tests to 10 positive tests
    3. 5 negative tests to 5 positive tests
    4. 50 negative tests to 1 positive test
    5. 10 negative tests to 5 positive tests
    1. Antigen testing
    2. Antibody titers
    3. PCR
    4. Lung biopsy
    5. CT scan

    Author of lecture COVID-19 (SARS-CoV2) Pandemic: Test Positivity Ratio and Death-to-test Ratio

     Raywat Deonandan, PhD

    Raywat Deonandan, PhD

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