With antibody testing on the rise, Texas is changing the way it reports the positivity rating -- the percentage representing the number of people who have tested positive for COVID-19 over a given period of time.
The positivity rating is one of a number of metrics used by state officials when they determined how to reopen the state amid the pandemic. In early to mid-April, numbers from the Texas Department of State Health Services showed positivity ratings in Texas were well over 10%.
When Gov. Greg Abbott announced his Open Texas plan in late April, the positivity rating had dropped to about 6%, and he cautioned that should the rating again climb to a sustained trend of around 10% that it would be a "red flag" that state leaders would have to look at.
Now, as the Open Texas plan rolls on, more attention is being given to the rating as an indicator of the continued spread of the 2019 novel coronavirus. Texas, as it turns out, is one of at least four states, including Vermont, Virginia and Georgia, that are combining numbers from two different tests, viral and antibody, to calculate the positivity rating.
Viral tests, performed through a nose or saliva swab, determine if a person is currently fighting the virus. Antibody tests, performed by a blood sample, look for signs a person has been exposed to the virus in the past but are not currently infected.
Some experts said combining the two can provide a misleading picture of the current spread of the virus and overstates the ability to test and track infections, a key consideration as the state eases restrictions put in place to curb the spread of the virus.
“What we've been requiring of labs is they report all tests together and so that's what we've been passing along and posting on our website,” said Chris Van Deusen, spokesperson for the Texas Department of State Health Services.
"No matter which way you look at it, with or without the antibody tests, the trend is the same and that it's generally going down, which is a good thing for Texas," Van Deusen said.
Until the last 10 days, antibody tests made up a small amount of the tests given. But with the recent increase in antibody testing, the number of antibody tests performed is growing statistically significant. So, beginning Thursday afternoon, the state began reporting a cumulative test number along with the antibody and viral tests separately.
The positivity rating, meanwhile, will now be calculated without including the antibody tests.
For Thursday, the positivity rating reported by the DSHS was slightly higher than the number reported in recent days (see the chart below), though this is expected with the removal of approximately 15,000 antibody tests.
When computing the positivity rating, the state is also taking an average of the number of positive tests from the last seven days and dividing it by an average of the number of tests given over the same period to determine the positivity rating. Van Deusen said the state is using the rolling average as opposed to just daily totals to smooth out the data and make it easier to spot trends.
“We want people to know exactly what's going on with the virus in Texas,” Van Deusen said. "To this point, the number of antibody tests has been small and it's not having a major impact on the positivity rate, certainly on the trend, on that at all. No matter which way you look at it, with or without those antibody tests, the trend is going down and that's a good thing."
The goal, Van Deusen said, is giving Texans the most accurate information possible in ever-changing conditions.