Antibodies test offered by local pharmacy (Assure) shows mixed results

[YSI]

My sibling and spouse recently tested positive for IgM and IgG antibodies.

They were minorly ill in November and experienced the classic loss of smell and taste.  They didn't see anyone at the time but when this cheap antibody test was offered 3 months later they were curious.  

That made ME curious, lol. I believe I was exposed to Covid on 3/13/2020 (where our state's breakout began.) However... I was also in my sibling's presence face to face in November without masks.

Frankly, I eat and drink so much I am not sure I would notice loss of smell and taste...but I did have the other symptoms of Covid last March, and since.

So my IgM was positive but my IgG was negative. I realize each one could be a false result.

Barring that, what conclusions can I draw from each result?

Is there a test for the age of my antibodies?

Do antibodies have a shelf life after which you might test negative?

What other viruses might this test have detected besides Covid? (I did have RSV-B 3 yrs ago.)

[AnnieBrooke]

I've only heard the answer to one of your questions above, and that is that  antibodies from having an actual case of Covid-19 don't necessarily show up if you wait too long to test.  I had a suspicious fever and illness in February, and in August or September looked into doing antibody testing to see if it had been Covid, and they told me that they couldn't guarantee it would show up even if it had been Covid.

[AnnieBrooke]

(By "show up," I mean of course that the test would be positive for antibodies to Covid-19.)

[Paxiled]

CDC says it's very uncommon for someone who has had covid to not test positive for both antigens, though obviously uncommon doesn't mean never.  The reliability of the test depends on which test was used and how heavily the area you're in was impacted with covid.  Nobody knows how long immunity lasts or even if there is immunity, though most believe there is immunity for some period of time for most people -- a few cases of reinfection have occurred, and the new more virulent mutations might pose much greater chances of reinfection but you weren't exposed to those.  November is pretty early for someone to have been infected in the US, that would have been China, though here too many believe the virus was circulating earlier.  Don't think how much you eat and drink would affect loss of smell or taste -- it's a disease, and you'd notice.  I would say your test is inconclusive because you didn't test positive for both, so you might want to try again and make sure the test is one of those with the high reliability.  But the results don't have any practical effect for you, it's just a matter of curiosity.  You could get reinfected anyway, some have, though it's rare as far as anyone knows.  You could still have it without symptoms as you're protected but still be able to pass it on, so you'd still have to obey all the current restrictions.  You could also run into one of the new variants which are much more virulent and therefore more likely to cause reinfection.  Until we reach herd immunity through vaccination of somewhere around 80% of the world's population, we're all still going to be wearing masks and social distancing.  Bottom line, as with most of this disease, who knows?