?A magnesium blood test can identify an abnormally low or high concentration of magnesium circulating in your blood.
But there is an important limitation.
Most magnesium in the body is stored inside cells and in bone, with only a very small proportion present in serum. This means serum magnesium is useful for detecting clinically important abnormalities, but it does not provide a perfect measurement of total-body magnesium stores.
A normal result therefore needs context, particularly if there are ongoing symptoms, gastrointestinal losses, kidney problems or medicines known to affect magnesium balance.
At the same time, it would be misleading to suggest that everyone with fatigue, muscle cramps or poor sleep has a hidden magnesium deficiency.
Those symptoms have many possible causes.
The purpose of a blood test for magnesium is to add objective laboratory information to the wider clinical picture.
London Blood Tests offers a private Magnesium (Serum) Blood Test for people who require a direct measurement of circulating magnesium, with clinic and home blood collection options available.
What Is a Magnesium Blood Test?
Magnesium is an essential mineral involved in hundreds of biochemical processes throughout the body.
It contributes to normal muscle contraction, nerve signalling, energy metabolism, bone function and regulation of cardiac electrical activity.
A standard magnesium blood test usually measures magnesium in the serum portion of the blood.
You may also see this described as:
· magnesium serum test
· magnesium lab test
· magnesium blood work
· Mg blood test
· blood test for magnesium levels
These generally refer to laboratory measurement of circulating magnesium.
The London Blood Tests Magnesium (Serum) Blood Test measures one biomarker: serum magnesium.
What Does a Magnesium Blood Test Actually Show?
A serum magnesium result tells you how much magnesium is circulating in the serum at the time your blood sample is analysed.
This can be clinically useful.
Low serum magnesium, known as hypomagnesaemia, can occur with gastrointestinal losses, poor absorption, certain medicines or increased loss through the kidneys.
High magnesium, known as hypermagnesaemia, is less common and is more likely to occur when magnesium intake is excessive in the setting of reduced kidney clearance or particular medical treatments.
However, serum magnesium is not equivalent to total-body magnesium.
Gloucestershire Hospitals NHS Foundation Trust notes that less than 0.5% of total-body magnesium is present in serum, even though serum magnesium remains the most widely used practical blood measurement.
That means the result is valuable—but it should not be expected to describe every magnesium store inside bone and tissue.
Can You Be Magnesium Deficient With a Normal Blood Test?
Potentially, but this needs careful wording.
A normal serum magnesium result indicates that the magnesium concentration circulating in the blood is within that laboratory's reference interval.
It does not directly prove that every intracellular or tissue magnesium store is optimal.
Because the body regulates circulating magnesium relatively closely, serum concentration does not perfectly correlate with total-body magnesium content.
However, that does not mean a normal serum result should simply be ignored or that an alternative test automatically proves a "hidden deficiency".
There is currently no single laboratory test that perfectly measures total-body magnesium status.
The most accurate interpretation combines laboratory findings with symptoms, medical conditions, medication, dietary history and, when appropriate, additional investigations.
This nuance is particularly important because online discussions of magnesium often move too quickly from:
"serum magnesium has limitations"
to:
"normal serum magnesium means nothing."
That is not correct.
Serum magnesium remains the standard and most widely available clinical measurement.
Why Is Serum Magnesium Still Used?
If serum magnesium has limitations, it is reasonable to ask why doctors and laboratories continue using it.
The answer is practicality and clinical usefulness.
Serum magnesium is readily measurable, well established and useful for identifying significant disturbances in circulating magnesium.
It is particularly relevant when investigating or monitoring situations such as gastrointestinal losses, kidney disease, low potassium, low calcium, certain medicines and suspected clinically significant magnesium imbalance.
It is therefore better to think of serum magnesium as:
a useful clinical measurement with recognised limitations
rather than:
a perfect measurement of total-body stores
or:
a useless test because most magnesium is intracellular.
Both extremes are misleading.
Do Routine Blood Tests Check Magnesium Levels?
Not necessarily.
Magnesium is not automatically included in every routine blood panel.
For example, a standard electrolyte or urea and electrolyte profile commonly focuses on markers such as sodium, potassium, chloride and bicarbonate.
The London Blood Tests Electrolytes Profile, for example, checks:
· Sodium
· Potassium
· Chloride
· Bicarbonate
Magnesium is a separate measurement.
So if you have had "electrolytes checked", do not automatically assume that magnesium was included.
Look at the individual biomarkers listed on your laboratory report.
How Is Magnesium Listed on a Blood Test?
On a laboratory report, magnesium may appear as:
· Magnesium
· Mg
· Serum Magnesium
The result is usually displayed alongside the laboratory's reference interval and measurement unit.
Do not compare your result with a reference range from an unrelated website.
Laboratory methods and intervals can vary.
The appropriate range is the one supplied with your actual result.
What Causes Low Magnesium?
A genuinely low serum magnesium concentration can have several possible causes.
Gastrointestinal Losses
Persistent diarrhoea, vomiting and some gastrointestinal diseases can reduce magnesium through poor absorption or direct loss.
Conditions causing chronic malabsorption can therefore affect magnesium status.
Medicines
Some medicines can contribute to low magnesium.
Long-term use of proton pump inhibitors has been associated with hypomagnesaemia in some people, and certain diuretics can increase renal magnesium loss.
Medication history is therefore relevant when a low result is being investigated.
Do not stop prescribed medicines because of an abnormal magnesium result without speaking to the clinician responsible for your treatment.
Kidney Losses
The kidneys play a major role in regulating magnesium.
Some conditions can cause excessive magnesium loss through urine even when dietary intake is reasonable.
Poor Nutritional Intake
Very restricted diets, malnutrition or prolonged inadequate intake may contribute, particularly when combined with other risk factors.
Alcohol-Related Problems
Chronic excessive alcohol intake can be associated with several electrolyte and nutritional disturbances, including magnesium depletion.
Other Electrolyte Abnormalities
Low magnesium can coexist with low potassium or low calcium.
In some cases, persistent hypokalaemia or hypocalcaemia can be difficult to correct until magnesium deficiency is also addressed.
This is one reason magnesium may be added when other electrolyte abnormalities persist unexpectedly.
What Are the Symptoms of Low Magnesium?
Mild magnesium deficiency may cause no obvious symptoms.
As magnesium falls further, possible features can include muscle weakness, cramps, tremor, twitching, tingling or other neuromuscular symptoms.
More significant abnormalities can affect cardiac electrical activity and contribute to arrhythmias.
However, these symptoms are not specific to magnesium deficiency.
Muscle cramps can also occur with dehydration, exercise, neurological conditions, other electrolyte abnormalities and many other causes.
Fatigue is even less specific.
That means symptoms may justify investigation, but they cannot diagnose deficiency.
If tiredness, stress or broader hormonal symptoms are the main concern, our guide to blood tests for stress, fatigue and hormonal imbalance explains other biomarkers that may also need consideration.
Can Low Magnesium Cause Muscle Cramps?
Low magnesium can contribute to muscle cramps, spasms and neuromuscular irritability.
But it is not the only explanation.
Potassium, calcium, hydration, exercise, medication and neurological factors can also influence muscle symptoms.
This is why ordering only magnesium because you have leg cramps may provide an incomplete picture.
Where electrolyte imbalance is genuinely suspected, magnesium is often interpreted alongside other laboratory results rather than on its own.
Can Low Magnesium Affect the Heart?
Magnesium contributes to normal electrical conduction and cardiac rhythm.
Significant hypomagnesaemia can increase the risk of cardiac rhythm disturbances, particularly when other electrolyte abnormalities such as low potassium are also present.
This does not mean an occasional palpitation proves magnesium deficiency.
Palpitations have many possible causes.
New, persistent or severe palpitations—especially when associated with chest pain, fainting, severe breathlessness or marked dizziness—should be medically assessed rather than investigated solely through a private magnesium test.
What Does High Magnesium Mean?
High serum magnesium is considerably less common than low magnesium in people with normal kidney function.
The kidneys normally remove excess magnesium efficiently.
Hypermagnesaemia becomes more likely when kidney function is significantly impaired, particularly when magnesium-containing supplements, medicines or other preparations are also being used.
Very high magnesium concentrations can affect blood pressure, reflexes, muscle function and cardiac conduction.
If a magnesium result is substantially elevated, particularly in someone with kidney disease or concerning symptoms, it warrants appropriate clinical review.
Simply stopping or changing treatment independently is not advisable.
Magnesium and Kidney Function
The kidneys are central to magnesium balance.
When the body needs to conserve magnesium, renal excretion can decrease.
When magnesium intake is high, functioning kidneys usually increase excretion.
This is why both unusually low and unusually high magnesium results may sometimes require assessment of kidney function.
For people who require simultaneous investigation of kidney and electrolyte markers, London Blood Tests also offers a Liver and Kidney Function Test, while magnesium itself can be ordered separately where clinically relevant.
Serum Magnesium vs Whole Blood Magnesium
London Blood Tests offers both a Magnesium (Serum) Blood Test and a Magnesium (Whole Blood) Test.
These tests assess magnesium in different blood compartments.
Serum testing measures magnesium in the fluid component of blood and remains the routinely used clinical measurement.
Whole-blood testing includes magnesium associated with blood cells as well as the circulating component.
Because most magnesium in the body is intracellular, cellular or whole-blood measurements are sometimes considered when serum results do not appear to explain the clinical picture.
However, this should not be simplified into:
"whole blood is always better than serum."
No single magnesium test perfectly measures total-body magnesium status, and alternative intracellular measurements are less standardised than routine serum testing.
If symptoms continue despite a normal serum result, the appropriate next step may be broader clinical assessment rather than automatically repeating magnesium using a different methodology.
What About a Red Cell or Intracellular Magnesium Test?
Red-cell magnesium is another approach sometimes used to assess intracellular magnesium.
Again, the theoretical attraction is clear: most magnesium is located inside cells rather than circulating freely in serum.
But this does not eliminate all the limitations of magnesium-status assessment.
Reference methods and clinical interpretation are less standardised than routine serum magnesium.
London Blood Tests' Mineral Screen (Whole Blood) assesses red-cell magnesium alongside several other minerals including zinc, selenium, copper, potassium and calcium.
A broader mineral profile may be relevant in selected circumstances, but a larger panel is not automatically necessary for everyone experiencing non-specific symptoms.
Testing should remain targeted to the reason for investigation.
Magnesium and Potassium: Why Are They Often Considered Together?
Magnesium and potassium are separate minerals, but their regulation can interact.
Significant magnesium deficiency can contribute to renal potassium loss, which can make low potassium more difficult to correct.
This is one reason clinicians may check magnesium when hypokalaemia persists or recurs.
London Blood Tests offers a standalone Potassium Blood Test as well as the broader Electrolytes Profile.
However, magnesium is not automatically part of that electrolyte panel and needs to be requested separately when required.
Do You Need to Fast for a Magnesium Blood Test?
Fasting is generally not required for a standalone serum magnesium test.
You can normally eat and drink before blood collection.
Normal hydration is encouraged.
However, supplements can influence magnesium levels, so the instructions supplied for your particular test should be followed.
London Blood Tests currently advises avoiding vitamin and mineral supplements for approximately 24–48 hours before testing where appropriate, unless you have been advised differently by a healthcare professional.
Do not stop prescribed medication without medical advice.
If magnesium is being collected as part of a larger blood profile, another biomarker in that profile may require fasting even though magnesium itself does not.
Should You Stop Magnesium Supplements Before Testing?
Whether supplements should be withheld depends on why the test is being performed.
If the goal is to understand your unsupplemented magnesium status, recent supplementation could affect the result.
If the purpose is to monitor treatment prescribed for a known deficiency, the clinician may want testing performed under different conditions.
London Blood Tests generally advises avoiding mineral supplements for 24–48 hours before testing unless otherwise directed.
If you are taking magnesium therapeutically or have been prescribed it because of a medical condition, check before changing your dosing schedule.
What Happens After a Low Magnesium Result?
The appropriate next step depends on the degree of the abnormality and the likely cause.
Mildly Low Magnesium
A modest abnormality may prompt review of diet, medicines, gastrointestinal symptoms and other electrolyte results.
Repeat testing may sometimes be useful.
Low Magnesium With Diarrhoea or Malabsorption
The cause of the gastrointestinal problem may need assessment rather than simply replacing magnesium indefinitely.
Low Magnesium With Low Potassium or Calcium
These abnormalities may be related and need to be interpreted together.
Low Magnesium With Medication Use
Medicines should be reviewed with the prescribing clinician, particularly when a treatment known to affect magnesium is being used.
Markedly Abnormal Magnesium or Significant Symptoms
Substantial abnormalities, severe weakness, cardiac symptoms, fainting or significant neurological symptoms require appropriate medical assessment.
A private blood test should not delay urgent care when symptoms are severe.
Should You Take Magnesium Supplements Based on Symptoms Alone?
Not necessarily.
Magnesium supplements are widely available, but symptoms such as fatigue, headaches, poor sleep and muscle cramps are not specific enough to prove deficiency.
Excessive supplementation can also be problematic, particularly in people with impaired kidney function.
Testing can be useful when there is a genuine reason to suspect abnormal magnesium.
Even then, supplementation should be proportionate to the result, symptoms, diet and medical context.
The objective is not to push a laboratory number as high as possible.
It is to maintain appropriate magnesium balance.
What Does the London Blood Tests Serum Magnesium Test Include?
The London Blood Tests Magnesium (Serum) Blood Test measures one biomarker:
Serum Magnesium
The current listed price is £38, excluding the relevant sample-collection charge.
The test has a listed laboratory turnaround of approximately one day.
Blood can be collected at an available clinic or through home or hotel phlebotomy.
The test is particularly straightforward when circulating magnesium itself is the marker required.
Where symptoms or clinical history raise broader questions about electrolyte or mineral status, additional testing may be appropriate rather than interpreting magnesium alone.
Private Magnesium Blood Testing in London
London Blood Tests offers private magnesium blood testing in London with clinic and home blood collection options.
The Magnesium (Serum) Blood Test provides the standard circulating magnesium measurement.
A Magnesium (Whole Blood) Test is also available where a different blood compartment has specifically been requested.
For people requiring a wider mineral assessment, the Mineral Screen (Whole Blood) includes magnesium alongside red-cell zinc, selenium, copper, potassium, calcium, chromium and manganese.
The largest panel is not automatically the best choice.
The most useful test is the one that answers the clinical question while avoiding unnecessary investigation.
Frequently Asked Questions About Magnesium Blood Tests
What is a magnesium blood test?
A magnesium blood test measures magnesium circulating in the blood. The standard test usually measures serum magnesium.
How is magnesium listed on a blood test?
It may appear as Magnesium, Mg or Serum Magnesium on the laboratory report.
Do routine blood tests check magnesium?
Not always. Magnesium is not automatically included in every routine blood or electrolyte panel and may need to be requested separately.
Can a blood test detect magnesium deficiency?
A low serum magnesium result can identify hypomagnesaemia. However, serum magnesium does not perfectly reflect total-body magnesium stores.
Can magnesium deficiency exist with a normal serum result?
Potentially. Most magnesium is located inside cells and bone, so serum concentration does not perfectly correspond with total-body magnesium. A normal serum result still provides useful clinical information and should not simply be dismissed.
What causes low magnesium?
Possible causes include gastrointestinal losses, malabsorption, certain medicines, renal magnesium losses, poor nutritional intake and chronic excessive alcohol use.
Can low magnesium cause cramps?
Yes, significant magnesium deficiency can contribute to muscle cramps, spasms and neuromuscular symptoms, although cramps have many other possible causes.
Can low magnesium affect potassium?
Yes. Significant magnesium deficiency can contribute to potassium loss and may make hypokalaemia more difficult to correct.
What causes high magnesium?
High magnesium is less common and is particularly associated with reduced kidney clearance combined with high magnesium intake, supplementation or magnesium-containing medicines.
Do you need to fast for a magnesium blood test?
Usually not for a standalone serum magnesium measurement. Follow the instructions supplied for your complete test order.
Should magnesium supplements be stopped before testing?
Recent supplementation can influence results. London Blood Tests generally recommends avoiding mineral supplements for 24–48 hours before testing unless otherwise advised.
Is whole blood magnesium better than serum magnesium?
Not universally. Whole-blood testing assesses a different blood compartment, but no single test perfectly measures total-body magnesium status. Serum magnesium remains the widely used routine clinical test.
What blood tests are useful alongside magnesium?
Depending on the reason for testing, potassium, calcium, kidney function and other electrolyte or mineral markers may provide useful additional information.
The Bottom Line
A magnesium blood test is useful for identifying abnormal circulating magnesium, but the result needs to be understood correctly.
Serum magnesium remains the standard, practical clinical measurement.
Its limitation is that only a very small proportion of total-body magnesium is found in serum. A normal result therefore does not perfectly measure magnesium throughout every tissue in the body.
That does not make serum testing useless.
And it does not automatically make alternative intracellular tests superior.
The most useful interpretation considers the laboratory result alongside symptoms, medication, gastrointestinal health, kidney function and other electrolyte abnormalities.
For private testing in London, London Blood Tests offers a Magnesium (Serum) Blood Test, a Magnesium (Whole Blood) Test and a broader Mineral Screen when additional mineral measurements are appropriate.
The goal is not to prove that every non-specific symptom is caused by magnesium.
It is to determine whether an identifiable magnesium imbalance is genuinely part of the clinical picture.
Independent Medical Sources
Gloucestershire Hospitals NHS Foundation Trust — Magnesium (Mg)
NIH Office of Dietary Supplements — Magnesium: Health Professional Fact Sheet
London Blood Tests Services Mentioned