The CDC is conflating viral and antibody tests. Here’s what you need to know

The government’s disease-fighting agency is compromising a few crucial metrics that governors depend on to reopen their economies. Pennsylvania, Georgia, Texas, and other states are doing the same.

The Atlantic reports today that the Centers for Disease Control (CDC) is “conflating the results of two different types of coronavirus tests, distorting several important metrics and providing the country with an inaccurate picture of the state of the pandemic.”

Several states have been doing the same: Georgia, Texas and Pennsylvania for example are reporting conflated testing numbers. As a result, some states are making decisions about when, where, and what to open based on bad testing data and inflated numbers, presenting a mirage of progress where in fact, there may be little or none.

It’s incomprehensible that the CDC’s scientists would make such a rookie mistake that muddies their own data— unless politics is interfering with their ability to do their job. This move by the CDC raises concerns about interference by non-scientists eager to raise their perceived testing rate following pressure from public health experts demanding higher testing rates to drive reopening.

Here’s what you need to know:

• The CDC is counting together two very different tests related to Covid-19: virus testing and antibody testing. Virus testing is diagnostic, indicating whether a person is currently infected. It tells you the state of play of the infection in the community: how many people currently have the disease and can potentially spread it.  The antibody tests, on the other hand, assess immune system response. They are the “rearview mirror” that tells you where you’ve been: how many people were previously infected.
• Virus testing allows us to trace the current trajectory of the disease and understand whether social distancing policies are working. Public health experts have been calling for wider expansion of this type of test to capture individuals with active disease before they spread it to others.  This is why the test-positivity rate is important— a lower proportion of tests coming back positive indicates that we are testing a broader swath of the population, not just those who are sickest. In contrast, data on antibody testing reveals information about the total prevalence of the disease in the population— but teaches us nothing about the current state of the disease or our testing capacity.
• Combine data on both of these tests, and the data lose meaning.
• Our team at HGHI has calculated that the U.S. needs to conduct 900,000 tests per day to safely reopen. Currently, data suggests that the US is testing about 400,000 people per day. While this is still far from our needed goal, it is double the amount of testing compared to a month ago, suggesting that we have made real progress on testing. Today’s reporting completely undermines that data and evidence of real progress.
• And, the percentage of tests that are positive, another key metric, has also improved substantially in reports from the CDC. Again, all of this has been seen as good news – that we are advancing toward the level of testing we need to open up our economy safely – when in fact we’re not.

Clearer and more standardized instructions on data collection from the CDC could have helped prevent this.