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.