Is Micronutrient Testing Accurate?

Oct 17 / Drs. Bryan & Julie Walsh
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Have you ever wondered about the accuracy of micronutrient testing?

You know, the laboratory tests that evaluate a patient’s vitamin and mineral status.

They sound incredible, but are they too good to be true?

We investigated the accuracy of micronutrient testing and found some interesting information.

In this article, we're going to focus on what we found on one of the more well-studied and popular micronutrients in the Functional and Nutritional Medicine industry - magnesium.

But first, let’s cover some basics.

What Are Micronutrients?

Quite simply, the word micronutrient refers to all the vitamins and minerals necessary for the human body to function properly. Everything from all the B-vitamins, vitamin C, and vitamins D, E, and K, as well as the minerals calcium, magnesium, zinc, iron, copper, selenium, molybdenum, and even boron. Each of those is a micronutrient. Micro means small, and nutrient is something needed by the body for growth, repair, or metabolism.

Why Are Micronutrients Important?

Micronutrients are critical for the optimal health of every single cell in the body. Many practitioners talk about the importance of carbohydrates, proteins, and fats, but fail to mention that, using carbohydrates, proteins, and fats in the body requires micronutrients.
In other words, without optimal levels of micronutrients, nothing works properly.

Our cells cannot make energy properly, we can’t detoxify or get rid of wastes properly, our brain won’t make neurotransmitters properly, hormones won’t work properly – micronutrients are the unsung hero in biochemistry, physiology, and overall wellness.

What is Micronutrient Testing?

Given the importance of micronutrients, laboratory tests have been developed to identify if someone is low in one, or a number of, micronutrients. After all, given the fundamental importance of micronutrients, if someone is low in a number of vitamins and minerals, identifying and correcting those deficiencies will likely do wonders for their health.

How to Test For Micronutrient Deficiencies

Unfortunately, there is no one way to test for all micronutrients at one time or in one tissue.

I know, bummer, right?

The truth is, some are better tested in the hair, some in blood, some in urine, some in cells, etc.

But, in terms of a single, comprehensive test, that accurately identifies a patient's micronutrient deficiencies - so that you know exactly what vitamins to give them - sorry, it does not exist.

To take a better look at the different ways of testing for micronutrient deficiencies, and how accurate they may be, let’s take a look at one of the most popular micronutrients in the Functional and Nutritional Medicine industry – magnesium.

How to Test for Magnesium

There are a variety of ways that have been used to evaluate magnesium status in humans. Here is a list of the most common ones:


Serum magnesium


Red Blood Cell (RBC) magnesium


Ionized magnesium


Urinary magnesium


Fecal magnesium


Hair magnesium


Mononuclear cell magnesium (eg lymphocytes)
Each of these has been used in the scientific literature to evaluate human magnesium status, with very conflicting results.

Gold Standard Magnesium Testing

There are two tests that have been cited as the “gold standard” for evaluating magnesium status.

They are:


Skeletal muscle biopsy – This test involves taking a sample of skeletal muscle tissue and evaluating its level of magnesium. 


Magnesium Loading (Retention) Test - This test often requires a hospital stay. There are different protocols for this. In general, a patient receives an intravenous (IV) dose of magnesium over a period of approximately 8-12 hours. Then, a 24-urinary test is done to see how much magnesium was excreted, and how much was retained. Typically, if approximately 20-30% of the magnesium that was given intravenously is retained, the patient is considered to be magnesium deficient.  

Is Magnesium Testing Accurate?

Standard laboratory tests for magnesium are not accurate.

Let’s take a look at a few examples.
  • Serum Magnesium – There are a few studies evaluating hypertensive patients taking diuretics (thiazides) which found that serum magnesium was normal, or even elevated, despite low levels of magnesium in mononuclear cells. This alone suggests the possibility of normal serum magnesium levels, despite deficient intracellular levels. 
  • RBC Magnesium – This is one of the more popular ways to evaluate magnesium in the Functional and Nutritional Medicine industry, despite the fact that that there are significant issues with it as a reliable marker of magnesium status. For example, there aren't any long-term studies using RBC magnesium, too few randomized clinical studies, and not enough studies comparing RBC magnesium to something more reliable, such as the magnesium loading test or skeletal muscle magnesium. Not to mention that blood contains a mere 0.8% of the body's stores of magnesium, yet is somehow supposed to accurately reflect the remaining 99.2% of magnesium in the body.
  • Urinary Magnesium – In a study evaluating almost 100 people experiencing symptoms of chronic fatigue, almost half of them were determined to be magnesium deficient using a magnesium intravenous loading test. Despite this, the mean values of serum magnesium, red blood cell magnesium, and urinary magnesium showed no significant difference between people with or without magnesium deficiency.
  • Ionized Magnesium – Despite the fact that is it used far less often as a marker of magnesium status, in a study evaluating critically ill patients, ionized magnesium was considered to be an insensitive marker of functional magnesium deficiency when compared to the magnesium loading test.
Beyond that, even skeletal muscle biopsy, which is considered to be one of the gold standard tests for magnesium deficiency, does not accurately reflect someone's magnesium status in every tissue of the body, such as the brain or heart. 

How to Accurately Evaluate Magnesium Status

There's good news and bad news.

The bad news is, testing magnesium status using laboratory testing methods is not accurate or reliable, nor is it a good way of determining whether or not a patient needs magnesium.

The good news is there are evidence-based alternatives. 


"I LOVE everything about these presentations. It makes me excited to practice. 😊"


"This is incredible information Dr Walsh."


"Another outstanding presentation...
Thank you!"


You ground me from all the FM Hype out there, which is mostly messy, biased, and FOMO driven. Please keep doing what you're doing. Your work is benefiting so many patients around the globe. Truly blessed to be amongst your students. Much love ❤️