I'm curious how the tests compare to each other. Because the child with 594 had 123 in January on the test where over 15 is positive. So did the antibodies go up or down?
For the Jan test, the threshold for positive was 15, and for the June test it was 0.8, so these are apparently different tests, produced by different manufacturers, using different scales.
Imagine you found one of those old fashioned thermometers with a glass tube containing mercury, attached to a piece of plastic, but it's so old that the numbers and scale printed on the plastic have been rubbed off. The mercury still rises with increases in temperature, so you can see that it's warmer today than yesterday, but you don't know the exact temperature. You lose that thermometer, but soon find another with the same problem. Again, it can show relative changes in temperature over time, but you can't compare these values with those measured on the first thermometer because they may be using different scales, with different minimum and maximum and a different spread of the scale. To make these thermometers useful, you could make a series of cups of water of known increasing temperature, place the thermometer in each one and re-create the scale on the thermometer.
That's where we stand with the antibody tests. We know that there's some internal validity, that is, 500 on one test means more antibodies than 100 on the same test, but we don't know how that compares to another test. Scientists are trying to standardize the tests, but AFAIK they're not there yet. I found one preprint from March 2021
Anti-Spike protein assays to determine post-vaccination antibody levels: a head-to-head comparison of five quantitative assays
https://www.medrxiv.org/content/10.1101/2021.03.05.21252977v1that concluded "Although all assays evaluated showed good correlation, readings from different assays were not interchangeable, even when converted to BAU/mL using the WHO international standard for SARS-CoV-2 immunoglobulin. This highlights the need for further standardization of SARS-CoV-2 serology."
So we can't really convert from one test to another yet. If you knew the brand of the first test, you might be able to find out the range that was found in general (that is, did go from 15 to 130, or from 15 to 1000), and get a sense of whether 123 in the high or low part of the range. And then do the same for the current test. But as you can see this would just give you an approximate comparison between the two values.
You want to know Did the antibodies go up or down? That we can probably answer from knowing how antibodies change in general. After an infection, it can take a few weeks for antibodies to reach a high level, where they remain for a while. After the infection is gone, it would be a waste of energy to continue to make so much of an unneeded molecule, and the antibody level gradually decreases. So after an infection in November, you'd expect high levels in January, which then decrease somewhat by June. But even though antibodies in the blood decrease, the cells that make those antibodies remain, and they can quickly ramp up production of antibodies when again exposed to the virus. It is possible that the child was exposed to the virus again, and had no observable symptoms, but did give the immune cells a boost to stimulate their antibody production a bit.
So now my question to you,
@Bookish : Why are you so interested in the antibody level?