Serum magnesium

Serum magnesium is the standard magnesium blood test, and it usually can't tell you whether you are short: the body holds blood levels steady by drawing on bone and cell stores, so the number stays normal until a shortfall is severe. Red-blood-cell magnesium is often sold as the better test, but no marker has been shown to beat the others, and the push to raise the cutoff for "low" is contested.

Read a normal serum magnesium as a floor, not as proof your stores are full, and don't supplement to push the number upward. No better routine test exists, so whether you need more is mostly a question of diet and risk factors such as diuretics, heavy exercise and older age.

Why the standard test misses a shortfall

Serum magnesium is the amount of magnesium in the liquid part of your blood, the figure a standard magnesium blood test reports. When intake falls short, the body defends that blood level by pulling magnesium out of bone and out of cells, so the serum figure stays inside the normal range while stores drain. By the time serum magnesium drops, the deficit is severe.

That makes the test insensitive for detecting inadequate body stores: a low result signals a deficit that is already severe, but a normal one rules out much less than it seems to.

The fight over where "low" begins

The reference ranges themselves are disputed. The MaGNet Global Magnesium Project, convened from an advocacy organisation, the Center for Magnesium Education and Research, argues that common lab cutoffs understate deficiency and proposes 0.85 mmol/L as the threshold for low magnesium.^[1]

Twelve nephrologists published a formal rebuttal. On a 0.85 threshold about two-thirds of US adults would be classed deficient, which is the reason to treat the proposal as contested rather than settled.

Red-blood-cell magnesium isn't proven better

Red-blood-cell magnesium, which measures the magnesium inside red cells, is often promoted as the more sensitive alternative. That superiority has not been demonstrated.

The only systematic review of magnesium status markers found that serum, red-cell and urinary magnesium all responded to changes in intake, without showing any one to be better than the others.^[2]
The most recent review concludes that no single marker reliably reflects whole-body magnesium status.^[3]
A 2026 clinical review treats total serum magnesium as the most practical measure, with red-cell magnesium as an occasional add-on when serum is normal but deficiency is suspected. It notes that the red-cell test is technically demanding and its reference ranges are not standardised between laboratories.^[4]

Low is a warning; high is not a goal

A large health-records study points both ways. In 162,936 matched pairs of adults aged 50 and over, those with repeated readings below the usual lower cutoff had about 33% higher dementia risk. In a separate analysis of people without kidney disease, repeated readings above the normal range were also linked to higher risk, by about 25%.^[5]

These are observational records from an older population, so illness that both shifts magnesium and raises dementia risk cannot be ruled out. Either way, it gives no reason to chase the number upward: above the normal range, readings were linked to higher risk too.

The score meant to replace the blood draw

The Magnesium Depletion Score is a simple clinical score built from kidney function, use of diuretics and of acid-reflux drugs (proton pump inhibitors), and alcohol intake. It was developed as an alternative to a blood draw.

Its validation is thin. In the one study that tested it, in 77 adults already at high risk of deficiency, it identified deficiency only slightly better than serum magnesium did, and not clearly better than chance; the difference was never formally tested. In US population data a high score tracked with cardiovascular death, but its link with death from any cause was not statistically significant.^[6] Its strongest link with death is in cancer survivors, where people with high scores were also about 23 years older and sicker.^[7]

A higher score also tracks with lower levels of Klotho, a protein made mostly in the kidney that declines with age. But up to two of the score's points come from kidney function, so some of that link is built in.^[8]

Symptoms don't settle it either

The symptoms usually listed for low magnesium, such as muscle cramps, low-grade fatigue, irritability and occasional palpitations, are subtle and none is specific. Most healthy adults who eat some nuts, leafy greens and whole grains get enough. The case for a supplement is strongest in heavy exercisers, older adults, people on diuretics, and people drinking very-low-mineral filtered water.

What to do

Take a low serum magnesium seriously; by the time it drops, the deficit is severe.
Don't treat a normal result as the all-clear, and don't pay extra for a red-cell test expecting a truer answer.
Judge your need from your diet and the risk factors above: heavy exercise, older age, diuretics, very-low-mineral filtered water.
If you take an acid-reflux drug (proton pump inhibitor) long term, a supplement only partly offsets the magnesium it costs you; ask your prescriber whether you still need the drug. The dose and form advice is on Magnesium.
If you have kidney disease, don't supplement without your clinician, whatever your result: failing kidneys can't clear excess magnesium.