If your doctor orders a magnesium test, you will usually be choosing between two blood tests. A serum magnesium test measures the amount of magnesium in your bloodstream. An RBC magnesium test measures magnesium inside your red blood cells. The RBC test can add information, but it does not replace the serum test, and no single test measures all magnesium status. A symptom list or one number on a report cannot settle whether your magnesium is low, so use this guide to understand each test and what to ask before the draw.
What each test measures
WebMD’s magnesium test explainer describes the serum test as measuring the amount of magnesium you have in your bloodstream, drawn from a vein in your arm. It also notes that “Doctors don’t test for magnesium levels in routine blood tests.”
WebMD says more than half the magnesium in your body is stored in your bones and tissues. Cleveland Clinic says less than 1% of your body’s magnesium is in your bloodstream, so a standard blood test doesn’t give the full picture. A review on magnesium and sleep likewise notes that a large proportion of the body’s magnesium is sequestered in bones and cells.
The RBC test looks at a different pool. ARUP Laboratories’ test directory says “RBC magnesium results reflect the intracellular stores and general homeostasis of magnesium.” WebMD’s explainer says the RBC test “may be a more sensitive test for detecting low levels of magnesium than a regular magnesium blood test.”
| Test | What it measures | Range as printed by the source | Points the source makes |
|---|---|---|---|
| Serum magnesium | Magnesium in the liquid part of blood | 1.7 to 2.2 mg/dL (WebMD) | ARUP says plasma or serum is preferred for routine assessment of magnesium deficiency |
| RBC magnesium | Magnesium inside red blood cells | 3.6 to 7.5 mg/dL (ARUP) | ARUP lists a 1 to 4 day turnaround and calls the test a laboratory developed test |
WebMD’s explainer gives one serum range, 1.7 to 2.2 mg/dL. Labs set their own ranges, so ask which range your lab uses for your age and sex before you compare a result with any number online.
Does an RBC test solve the limits of serum testing?
Not fully. The Optimal DX Research Blog, published by a company whose blog promotes its own ranges, says RBC measurement “may provide additional information about what’s ‘in the pantry’ and may better reflect body stores.” ARUP takes a narrower view. Its page says RBC magnesium “may be useful in the assessment of tissue stores,” and that for routine assessment of magnesium deficiency, plasma or serum “is preferred.”
The RBC test has its own pitfalls. ARUP warns that “Results may be falsely low if RBCs in the submitted specimen are lysed or not promptly separated from plasma.” The Optimal DX blog notes the reverse problem: in severe hemolysis, “a significant amount of magnesium can be released into the blood, increasing serum levels.” Sample handling and blood cell breakdown can move either test, so ask the lab how it handles samples.
Are methods and reference intervals interchangeable?
No. Each lab sets its own reference interval and reports in its own units. ARUP tells readers to divide its mg/dL result by 2.43 to get mmol/L, so its RBC range of 3.6 to 7.5 mg/dL works out to about 1.48 to 3.09 mmol/L.
| Source | Sample | Range as printed |
|---|---|---|
| ARUP Laboratories | RBC | 3.6 to 7.5 mg/dL |
| Optimal DX blog, its “standard range” | RBC | 4.00 to 6.80 mg/dL (1.66 to 2.82 mmol/L) |
| Optimal DX blog, its “ODX Range” | RBC | 6.00 to 6.80 mg/dL (2.48 to 2.82 mmol/L) |
| WebMD explainer | Serum | 1.7 to 2.2 mg/dL |
| Quest Diagnostics, as quoted in a 2021 Optimal DX post | Serum | 1.5 to 2.5 mg/dL (0.62 to 1.03 mmol/L) |
| Labcorp, as quoted in the same post | Serum | 1.6 to 2.3 mg/dL (0.66 to 0.95 mmol/L) |
| Merck Manual, as quoted in the same post | Serum | 1.79 to 2.6 mg/dL (0.74 to 1.07 mmol/L) |
The cut-off also changes the count. The Optimal DX blog reports a 2014 study of 115 university students in Brazil, in which 87% would have been classed as magnesium deficient using a plasma cut-off of 2.07 mg/dL. Check which lab printed each range and which units it uses before you compare two reports.
What evidence supports the proposed clinical use?
The studies behind RBC testing are small, and most are not trials of treatment decisions. Here is what they report:
- A 1993 study in the Indian Journal of Medical Research, indexed by PubMed’s record of a 1993 seizure study, compared 100 patients with idiopathic generalised tonic clonic seizures with 95 matched healthy controls. The patients had significantly lower serum, CSF and RBC magnesium. Serum magnesium did not correlate with seizure frequency, control or duration, while low CSF magnesium did.
- A pilot study of 27 people with cognitive impairment and 21 without, cited by the Optimal DX blog as Sitzia 2020, found mean RBC magnesium of 3.14 mg/dL in the impaired group and 3.7 mg/dL in controls. The researchers noted that plasma magnesium did not identify deficiency.
- A cross-sectional study of 70 critically ill ICU patients, cited by the same blog as Heidari 2020, found a significantly increased risk of cardiac complications and mortality when RBC magnesium was below 4 mg/dL.
Because these are small or observational studies, they cannot settle whether an RBC result should change treatment in everyday care. They are a reason to ask your clinician why the test is being ordered, not a reason to order it yourself.
What hurts when your magnesium is low?
WebMD’s explainer lists tremors, muscle spasms, cramps and numbness in the hands and feet, abnormal eye movements, and fatigue and weakness. Severe low levels can cause seizures, confusion and abnormal heart rhythms. These symptoms have many causes, so treat them as a reason to ask about testing. For muscle-related context, see magnesium for muscle and nerve health.
What illness can cause low magnesium?
WebMD’s explainer lists chronic kidney disease, uncontrolled diabetes, alcohol use disorder, malabsorption syndrome, malnutrition, chronic diarrhea, and low potassium or calcium as reasons a doctor may want to monitor magnesium. Those are reasons for a clinician to look at your history, not a self-check.
What the marketing claims say, and how to read them
The Optimal DX Research Blog argues that standard ranges are too low. It says “Clinicians must look beyond standard reference intervals and not wait for a pathological diagnosis,” and it offers its own RBC range. It also cites what it calls an expert minimum of 6 mg/dL (2.47 mmol/L), attributed to a 2018 review by Razzaque in Nutrients. That figure comes from a different source than ARUP’s 3.6 mg/dL lower limit, so check where each number comes from before comparing them.
Treat any company range as that company’s position. Ask which lab or guideline it comes from before you use it to read your own report.
Magnesium test comparison and visit checklist
- Ask why this test is being ordered, and whether serum, RBC or a urine test is the choice.
- Ask what result would change your care.
- Ask which reference range and units the lab uses, and whether it matches your age and sex.
- Ask who will interpret the result alongside your history and your medicines.
- List your medicines and supplements. The sources name diuretics, certain antibiotics, proton pump inhibitors, antacids, laxatives, lithium, insulin and thyroid medicine as ones that can change magnesium. Talk to your prescriber before changing any medicine.
- Ask how the sample will be collected, labeled and transported, and how quickly it is processed.
- If you have chronic kidney disease, tell your clinician before any magnesium decision. WebMD’s explainer lists chronic kidney disease among the reasons a doctor may want to monitor magnesium.
- Ask the lab or your clinic’s billing office for the price and whether your plan covers it. Prices change, so check directly.
Practical alternatives and the next decision
Serum is the test ARUP recommends for routine assessment. WebMD’s explainer also describes a 24-hour urine collection, noting that levels can rise and fall with the food you ate that day, and a cheek-cell test it calls the magnesium EXA test. Ask your clinician which of these fits your situation and what they will do with the result.
Your next step is to write down your questions from the checklist, bring your medicine list, and ask who will interpret the result. If you want to understand the risks of taking too much magnesium, read magnesium overdose before you talk with your clinician.
What the evidence can and cannot establish
The research establishes that serum and RBC magnesium are measured in different parts of blood, that labs report different ranges in different units, and that some small studies found lower RBC magnesium in certain groups. It cannot tell you, from one number, how much magnesium your whole body holds. It also cannot identify a single gold-standard test. Here is the lab-listing status as of November 6, 2026: ARUP’s test directory lists this RBC test as a laboratory developed test and as approved by the New York State Department of Health. This is information, not legal advice. Check the directory entry for the lab your clinician uses.
Bottom line: ask your clinician why the test is proposed, what its limits are, and who will interpret the result. That conversation matters more than which tube goes to the lab.
By BackToHealthTroy.com Editorial Team