Test Positivity: Explained

Test positivity is the percentage of COVID testing that comes back positive. It is not the total number of positive tests. If a state or county has a high test positivity rate, it is a sign of insufficient testing in that area.

To make sure there are no unknown infections that could lead to an outbreak, states and counties need to test a large enough percentage of the population. Test Positivity also alerts us to possible surges in ICU headroom used.

Let’s look at the pictures below. Red represents a positive test, green a negative test, and grey untested. There are two positive COVID tests in each of these pictures. In the first picture (left), test positivity is 20% because two people out of 10 tested positive. There are 90 people left who remain untested and could be infected. Now, look at the third picture (right), where the test positivity is 2%. The entire population, 100 out of 100 people were tested. The closer we get to testing everyone, the less likely we are to miss positive cases.

3 panels showing testing variations where a A, 10 people are tested, with 2 positive tests (Tp=20%). B, 20 people are tested, with the same 2 positive tests (Tp=10%). A, 100 people are tested, with the same 2 positive tests (Tp=2%).
Red represents a positive test, green represents a negative test, grey represents someone untested

Here’s a video that helps explain test positivity:

How Does Covid Act Now rate this metric?

The ratings below are based upon recommendations from the World Health Organization, Harvard University epidemiologist William Hanague, Admiral Brett Giroir, and a member of the White House Coronavirus Task Force.

How do you know if your area is testing enough?

As Michael Ryan, executive director of the WHO Health Emergencies Program, says, “If 80-90% of the people test positive, you are probably missing a lot of cases.” The White House Coronavirus Task Force’s reopening guidelines also point to a low test positivity as a key metric for reopening. 

Countries like Taiwan, New Zealand, Australia, and South Korea were back to normal activity with test positivity rates below 3%. As of Aug 11, 2020, South Korea’s test positivity was 0.9%.

When test positivity decreases, we move from testing only the sickest people to those who are not infected, asymptomatic, or identified through contact tracing which is especially important since we know infected people can spread COVID with no symptoms.

It is critical, due to testing and reporting delays, that we get as much testing information as possible to track surges and act quickly to contain COVID.

Where does our data come from for this metric?

To calculate test positivity, Covid Act Now uses new positive cases and COVID death data from Corona Data Scraper.

Test positivity is just one of several important metrics to determine how well your state or county is doing in the fight against COVID. Learn more about daily new cases, infection rate, ICU headroom used, and contact tracing. For a description of assumptions and methodology, please see our references and assumptions document, along with our data sources presentation.

Check out our Youtube Channel for our educational videos.

To learn more about Covid Act Now, visit our about page. For more information please reach out on our contact page or by email at info@covidactnow.org.

Sign up to our alerts to stay up-to-date on the COVID risk level in your area.

Return to covidactnow.org


Recent Posts