One should do a full scale blood test with their GP before determining if they should NOT take a given supplement. Don't rely on youtube for this.
The video in OP is true in that the medical fields and journals are more focused on sensitivity as opposed to specificity. More on that here
https://en.wikipedia.org/wiki/Sensitivity_and_specificity This is primarily for liability reasons, better to get a false positive than a false negative. The rest of it i will not endorse at all.
Basically, under current models, 4 possibilities exist: you have a "condition" and we successfully identified it; you have a condition and we failed to identify it; you do not have a condition yet we misdiagnosed you as having one; you do not have a condition and we determined you dont have it too. Only empirical measurements can identify this, and never at a rate of 100% sensitivity AND specificty. Its always more like 80% vs 95%, or 75% vs 99%.
Sensitivity means exactly as it's implied: the degree to which it detects something. Specificity being the opposite, the degree to which it excludes something. In a perfect world, you hit 100% on both ends (this never happens in science).
The point: Never ever do a medical procedure (or supplement) with a mere two tests.
Two is the minimum standard. Do a 3rd or 4th. This minimizes harm on your end. Id rather be 1% wrong than 7% wrong, especially since insurance companies are terrified of false
negatives from a legal perspective (meaning they will always cover tests no matter what).
Relying on your own subjective measurements is a mere start. If you are symptom free, fine, do life, but the absence of symptoms, statistically speaking, means literally nothing. Likewise, the presence of symptoms also means nothing if it's only measured a couple times. Everyone likes to convince themselves they are some sort of fucking physiologist (with their lol-degree), yet fail to understand the actual numbers behind how measurements of a deficiency actually works mathematically. Nothing in medicine is exact, even in 2018. Maybe in 3018 it will be 99% accurate but i doubt it.