Antigen Testing Debate As Approval Sought For Faster Coronavirus Testing
The push to “re-open” economies is intensifying the need for more, and faster, testing for Coronavirus.
The tests currently used to see if people are actively infected with the virus are sophisticated molecular tests that can spot its genetic material in specimens swabbed from inside peoples’ noses and throats. Most use the technique known as polymerase chain reaction — PCR — to generate and analyze millions of copies of genetic sequences from the virus.
But those tests require sophisticated equipment, highly trained technicians and complicated processes that use specialized chemicals; those strict requirements of PCR tests, and problems in reliably meeting them, have all contributed to the testing shortage.
Widescale testing is considered crucial for determining the scope of the pandemic, bringing it under control and keeping large new outbreaks from occurring. Testing enables public health authorities to spot new infections quickly so patients can be isolated and so anyone they might have had contact with.
Typically, antigen tests are much easier and less expensive to manufacture and simpler to perform than either PCR tests or antibody tests. The same technology is used in pregnancy tests.
“It looks very much like a pregnancy test. And they’re very simple. They don’t require special chemicals or training to use them. And they can give you a read out in about 15 minutes or so,” says Gehrke, one of the collaborators on the E25BIO test.
Gehrke says his company’s test appears to be very accurate when evaluated with samples of the virus in the lab.“The difficult question — and the one we want to ask — is how well does it perform on samples from real patients,” Gehrke says.
Papenburg questions whether antigen test could ever be as reliable as the genetic tests because, unlike those PCR tests, they can’t produce large quantities of the target proteins — so might be more likely to miss an infection.
False negatives are gonna be a big problem with these. Because of the low protein levels in the samples. And this is one case where a false negative has more bad consequences than a false positive – false positive, the person gets sent home for a few days to ‘recover;’ false negative, the person could infect a bunch of people.
Eventually we might get to an antibody/saliva test that’s GOOD but I don’t think we are there yet. I’ve seen reports of sensitivity rates in the 80 percents and that’s TERRIBLE because it will unleash people who can be spreading the disease on the rest of the world. This is why I’m leery about the “open everything up now” calls some are making
(I always have to look up “sensitivity” vs. “specificity,” I am a biologist but not the medical kind; I think I used the correct term there – the one that detects true positives accurately – but it’s still early in the morning)Report
It might not be ideal, but currently, we don’t have enough of the PCR tests, can’t make them fast enough to satisfy the demand, and thus you have lots of people running around infecting others anyway (because folks are breaking quarantine).
The antigen tests at least increase the amount of knowledge we have, even if it’s not perfect.Report
Seems like a 15 minute test would be wonderful to secure nursing homes or other high risk places. What Illinois has started doing is training staff to perform the swab test, and they have a contract with a lab to perform 3000 tests per day, with results in 48 hours. I’m not sure what test frequency that implies, weekly for a portion of those in the facility? Given how quickly the infection appears to spread inside these facilities, that type of testing seems to rely a lot on timing and luck.Report
Because of the low protein levels in the samples. And this is one case where a false negative has more bad consequences than a false positive – false positive, the person gets sent home for a few days to ‘recover;’ false negative, the person could infect a bunch of people.
How much does this matter? If there’s, say, a 5% false positive rate, is that enough to keep R_t above unity?
For sensitivity and specificity, just think about the normal meanings of the words. Sensitivity is how sensitive a test is to detecting the disease; if it’s 90% sensitive, it will detect 90% of cases. Specificity is how specific positive results are to the disease; if it’s 98% specific, 98% of positive results will indicate an actual case.Report
It’s obviously useful as population screening. However, individuals will use the results to guide their behaviors, and the median member of the general public often doesn’t really understand probability that well. In other words, the risk is:
“I tested negative” -> “It’s safe for me to visit grandma.”
Ofc, public health officials can work to educate people, but how well does that work in general.
Summary: false negatives do matter. They can cause harm.Report
Sure… and yet… wouldn’t it be better to know with whatever degree of accuracy the tests can offer than to just not know?Report
The NYC metro area seems to have suddenly gotten access to widespread testing for both actively sick folks and those carrying antibodies. Because I had a high exposure risk (the boys’ mom contracted it and they were with her during her most contagious time and then with me for 16 days) but never contracted symptoms, I’ll be getting an antibody test later this week. It involves a blood draw and will get me “highly accurate results” within 3-5 days. I believe the active virus tests still involve the nasal swap. I can report back on the experience one it is complete.
https://www.citymd.com/news/covid-19-testing-update?utm_source=sfmc&utm_medium=email&utm_campaign=4_27+COVID-19+Testing&utm_content=COVID-19+testing+FAQ
This link is from CityMD, a major urgent care chain in the area. I spoke with my PCP whose office remains closed, but he can conduct a telehealth appointment to authorize a prescription for a test done at a lab. My sons’ pede can do the blood sample in office. This is all in Bergen County, NJ, one of the harder hit areas in the region.Report
From my time in a biochem lab I’ll tell you that a good rule of thumb with a new antibody based test is to assume it’s overhyped junk until proven otherwise.
Antibody-Antigen recognition is a chemically finicky thing that can be easily messed up by different conditions and frequently newly produced antibodies are as robust in recognition of their supposed target outside the manufacturers testing place. It isn’t nearly as robust and reliable of a biotechnology as PCR is.
Until something like this had been demonstrated to work reliably as advertised in the field and not on the bench, be skeptical.Report
My results came back negative. Here is what was included with that:
– Detection of IgG antibodies may indicate exposure to
– SARS-CoV-2 (COVID-19). It usually takes at least 10 days
– after symptom onset for IgG to reach detectable levels. An
– IgG positive result may suggest an immune response to a
– primary infection with SARS-CoV-2, but the relationship
– between IgG positivity and immunity to SARS-CoV-2 has not
– yet been firmly established. Antibody tests have not been
– shown to definitively diagnose or exclude SARS-CoV-2
– infection. Diagnosis of COVID-19 is made by detection of
– SARS-CoV-2 RNA by molecular testing methods, consistent
– with a patient’s clinical findings.
–
– This test has not been reviewed by the FDA. Negative results
– do not rule out SARS-CoV-2 infection particularly in those
– who have been in contact with the virus. Follow-up testing
– with a molecular diagnostic should be considered to rule
– out infection in these individuals. Results from antibody
– testing should not be used as the sole basis to diagnose
– or exclude SARS-CoV-2 infection or to inform infection
– status. Positive results could also be due to past or
– present infection with non-SARS-CoV-2 coronavirus strains,
– such as coronavirus HKU1, NL63, OC43, or 229E. This test
– is not to be used for the screening of donated blood.Report