Are Magnesium Lab Tests Accurate?

Magnesium lab tests may or may not be accurate.

It depends on whether you are getting yourself assessed for hypomagnesemia (low magnesium level) or hypermagnesemia (high magnesium level).

To continue our discussion on the accuracy of serum magnesium levels, we need to first define what normal magnesium levels are as accepted in clinical practice.

Normal magnesium levels in adults are 1.7 to 2.4 mg per deciliter (0.7 to 1.0 mmol per liter).

This range is tightly regulated through mechanisms involving intestinal absorption, renal excretion, and storage in bone.

Low magnesium levels, or hypomagnesemia, are defined as serum magnesium concentrations below 1.7 mg per deciliter (0.7 mmol per liter).

Hypomagnesemia can result from various causes, including inadequate dietary intake, increased gastrointestinal loss, reduced renal (kidney) reabsorption, or redistribution of magnesium from the extracellular to the intracellular space in the body's cells.

High magnesium levels, or hypermagnesemia, are defined as serum magnesium concentrations exceeding the upper limit of the normal range, which is typically 2.4 mg/dL (1.0 mmol/L).

Hypermagnesemia is relatively uncommon and usually occurs in the context of renal insufficiency (kidney disease or injury) or excessive or inappropriate intake of magnesium-containing substances, such as dietary supplements, antacids, or laxatives.

Now that we have defined normal, hypo, and hyper magnesium levels, let's explore whether these levels are accurate. 

As I mentioned earlier, the accuracy depends on whether you are assessing hypomagnesemia or hypermagnesemia.

A low serum magnesium level can be helpful but is not always accurate for diagnosing magnesium deficiency.

Here’s why:

• Magnesium RBC test (measures magnesium inside red blood cells)
• Magnesium loading test (checks how much magnesium is excreted after supplementation)

Clinical symptoms + risk factors (e.g., medications, chronic diseases, diet) are also important.

A low serum magnesium level usually means you are deficient, but a normal result doesn’t always mean you’re in the clear.

Always consider symptoms and context.

Symptoms of Hypomagnesemia are:

A high serum magnesium level is usually more reliable and accurate than a low one — but context still matters.

Since your body tightly controls magnesium levels, elevated serum magnesium (hypermagnesemia) usually means there’s too much magnesium in your blood, and your body can’t regulate it properly.

This typically happens when the kidneys aren’t working well and can’t get rid of excess magnesium.

• Accurate
• Clinically significant
• Worthy of attention, especially if accompanied by symptoms

• Kidney failure
• Excessive magnesium intake (e.g., from supplements or antacids/laxatives like magnesium citrate or milk of magnesia)
• Medications that affect kidney function (e.g., certain diuretics, ACE inhibitors)
• Adrenal insufficiency

• Nausea, vomiting
• Weakness
• Low blood pressure
• Slow heart rate
• Difficulty breathing
• Cardiac arrest (in severe cases)

Yes — high magnesium is usually accurate and clinically meaningful. If someone has elevated levels, it should be investigated and managed, especially if they have kidney issues or are taking magnesium-containing products. 

Reference:

Magnesium Disorders. Touyz RM, de Baaij JHF, Hoenderop JGJ. The New England Journal of Medicine. 2024;390(21):1998-2009. doi:10.1056/NEJMra1510603.