I can't speak for all nutrients, but it's nearly impossible to detect magnesium deficiency in blood tests. Presumably, other nutrients might be difficult to check as well. Which makes the tests misleading or downright wrong.

It should be part of the health system. For now in the UK we have lab services like this one: https://thriva.co/ (not affiliated). You send your blood by post and they analyse it.

Just measuring those 4 costs 83€ in a laboratory in Vienna. And there are more measurements one should probably take! And you need to test them more than once to see if supplementation works and you are not getting too much.

That's not cheap...

We're talking an expense of somewhere around 1k total over a period of a few months. While not within everyone's reach (and if a single check is performed say yearly after initial alteration of your diet also an ongoing expense), it's far from an insurmountable cost. Especially considering this directly impacts your day-to-day well-being.

If shouldered by the individual, an extra $1k over a few months, plus however much it costs to keep re-testing, and the cost of supplements or better food to correct the deficiency, is a staggeringly insurmountable cost for the people who need it most—the poor.

As you say, this directly impacts day-to-day wellbeing. It's exactly the kind of thing that should, if it's even moderately effective, be provided free of charge for every human being.

I agree with you, but I must also admit it's not much of a concern if you live in a country with solid health insurance.

I had a load of blood tests done in the uk and the health insurance company were charged over £1000. I couldn’t believe it!

Define "a lot"

I remember doing some tests once that had a cost breakdown and they would vary a lot in price (meaning some would be let's say, single digits, some others would be in the triple digits area - think USD/EUR price ranges)

The medical school at the university nearest me offers a range of such tests on a walk-in basis for prices ranging from around $15-40 USD each. Obviously, YMMV, but inquire with any medical schools near you that have research hospitals or training clinics.

For some nutrients it’s easy to test for, for others the tests are controversial with regards to accuracy.