Just to put this in some context, but the Quest Diagnostics and other tests currently available are good but not great. For example, from their website there are the following disclaimers:

This test has not been reviewed by the FDA.

Positive results may be due to past or present infection with non-SARS-CoV-2 coronavirus strains, such as coronavirus HKU1, NL63, OC43, or 229E.

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.

https://www.questdiagnostics.com/home/Covid-19/HCP/

Other sources state the following, however its important that not a lot of research has been done to validate the manufacturers specifications, of these tests such as specificity. Only over time and independent review will these numbers be more accurately quantified:

"The company started out using the Euroimmun AG Anti-SARS-CoV-2 ELISA IgG antibody test and then added the Abbott IgG antibody test. It is continuing to use both tests, although the Abbott test has become the primary platform. Euroimmun is a division of Perkin Elmer.

Both tests have received emergency use authorization from the FDA; only 10 serology tests have received this designation from the agency. The Euroimmun IgG test has a reported specificity of 96% and sensitivity of 65%, while the Abbott test claims a sensitivity of 100% and specificity of 99.5%. The FDA has just cracked down on serology tests requiring supporting validation documentation to be submitted within 10 business days of an initial notification."

Similar story here, had COVID-19-like symptoms in mid-March, but the Abbott test (administered at CityMD) came back negative.

It makes me wonder what exactly is going on with this test. Do some people not produce antibodies? Is it overly specific to one strain of the virus and other people in NYC got a different strain that doesn't produce compatible antibodies? Was there some non-COVID-19 respiratory infection that hit NYC at around that same time that went under the radar? Is this test somehow just not as good as we think it is?

> Was there some non-COVID-19 respiratory infection that hit NYC at around that same time that went under the radar?

Not really under the radar, there are always a lot of respiratory illnesses. In my area, after reducing non-covid cold/flu by an order of magnitude due to social distancing, we're testing everyone with cold/flu symptoms, and they're still ~96.5% not covid-19.

Yeah, but I'm in NYC. Since some inflection point in March the vast majority of patients presenting with these symptoms do in fact have COVID-19. Being the global epicenter of the pandemic will do that :/

Slim majority, not vast - NYC test positivity rate peaked at around 55% IIRC.

A lot of those tests were people not presenting with COVID-19 symptoms though. Think of all the asymptomatic people who were getting tested; that group had a much lower test positivity rate than 55%, thus bringing the average down. For people who had all the right symptoms, it was higher than 55%.

That effect doesn't seem like it would skew a larger difference between Alberta and NYC test positivity rates. If anything, that would depress Alberta test positivity rates more than NYC ones, since we've got more spare testing capacity for people without all the right symptoms.

What does Alberta have to do with anything?

It's the "we" (I live in Alberta) I was originally referring to with the 3.5% test positivity rate.

Keep in mind that many coronaviruses can cause loss of smell/taste.

But which other coronaviruses are widely circulating right now is the question. As in, if you were to lose your smell because of a coronavirus, how likely is it that it was not SARS-CoV-2?

I don't know the absolute figures in terms of cases per year, but the four common coronaviruses are... common. From [1], we see that 1 in 200 flu-like infections was the coronavirus HKU1. This [2] paper corroborates and gives figures for the other three common viruses.

We can therefore assume that about 1 in 25 bad-cold or flu-like illnesses are caused by a coronavirus.

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5466428/

Pretty much any URTI can cause loss of taste. The loss of smell causes loss of taste.

I also signed up for the test. Should be interesting.