Insights: Why Tests Fail

LexaGene CEO Dr. Jack Regan featured in Science News email update 24 April 2020. Here is an excerpt from the newsletter featuring Dr. Regan’s insights:

Q&A: When the tests fail

To find out why fast diagnostic tests for COVID-19 sometimes fail, Science News talked to Jack Regan, a virologist who founded LexaGene, a company based in Beverly, Mass., that is developing a test that would detect COVID-19, influenza and other common respiratory viruses at the same time. The interview has been edited for brevity and clarity.

SN: Why does it matter if a small percentage of people get a false negative result?

Regan: There are reports that the Abbot ID NOW test, touted as giving a result in as little as five minutes, has a sensitivity of only about 85 percent. That means if you test 100 people, you’re going to send 15 of them home thinking that they’re not infected when they really are.

That is a big problem … [because] we can’t be sending 15 people home telling them they’re not sick, or sending them to a COVID-19-free ward at the hospital where they could infect other people.

SN: Why do these fast diagnostic tests sometimes give false negatives?

Regan: Everybody wants their cake and ice cream, too. They want very, very high quality data and they want it instantaneously. Because companies are prioritizing speed, they often skip a critical step called sample prep. It’s the process of taking a [nose swab sample, for example,] and purifying the genetic material in it prior to doing an analysis. The reason that’s important is because there are numerous chemicals and compounds in a sample that can inhibit chemical reactions in the tests that are necessary to give an accurate response.

For instance, people who are sick are often on a lot of medications, and chemicals in those medications can inhibit the reactions. Smoking or drinking coffee that day might have something that can inhibit a reaction. So if you skip sample prep, you make yourself more susceptible to getting a false negative.

And genetic material likes to create structures that are more stable. PCR genetic tests, which are considered the gold standard for diagnostic tests but take longer to give a result, use cycles of higher and lower temperature to melt the structures and make the genetic material more accessible. If you’re operating at a single temperature, which many of the rapid tests do, those structures can be harder for a test to unfold and read. This can contribute to the failure of the chemistry.

SN: Are there concerns about diagnostic tests you could do at home?

Regan: Sample collection is key for getting an accurate result. If you ask people to do it themselves, they’re going to stick a nasal swab up just past the nasal opening an inch or inch and a half. In reality, you want to stick that swab in four or five inches and rub the inside of the nasal cavity. It’s really uncomfortable. This requires a lot of training in a health care setting to get right.