Blood tests are pretty useless for magnesium. The deficiency is in cells, magnesium level in blood is not a good test for that deficiency.

They have blood tests for both (serum or RBC):

The Magnesium RBC Test measures magnesium inside red blood cells, providing a more accurate assessment of magnesium status than serum tests.

m_mueller 2 hours ago | parent | prev | next [-]

wouldn't a cell deficiency lead to it depleting also the magnesium levels in the blood, simply based on osmosis?

	▲	manoDev 2 hours ago \| parent \| next [-]
		The organism dumps Mg on the blood as much as necessary because it needs to remain in stable concentrations. When a serum Mg test comes out low, the patient already has severe deficiency and visible symptoms, so isn’t very useful unless you’re investigating a systemic issue. As explained by my endocrinologist.
	▲	stouset an hour ago \| parent \| prev [-]
		If blood concentrations drop, you die.

what is a good test for magnesium deficiency?

	▲	petilon 2 hours ago \| parent [-]
		They only have blood tests, and that's not very useful.