Magnesium (serum or urine)

The test measures magnesium concentration in serum or urine. It is performed for suspected magnesium deficiency or excess, for example with muscle cramps and tremor, heart rhythm disturbances or low potassium and calcium, and with medicines and diseases affecting magnesium.

Who the test is for

• people with muscle cramps and twitching, tremor, weakness or tingling;
• people with heart rhythm disturbances;
• people with low blood potassium or calcium;
• people taking diuretics or stomach acid-reducing medicines (proton pump inhibitors) long term;
• people with persistent diarrhoea or poor absorption, excessive alcohol intake or poorly controlled diabetes;
• people with kidney disease, particularly if taking magnesium preparations.

What magnesium is and what the test shows

Magnesium is a mineral needed for muscle and nerve function, heart rhythm, cellular energy production and bone health. Most is in bones and inside cells, while blood contains less than 1 % of total body magnesium.

Serum magnesium therefore poorly reflects total stores. A low result suggests deficiency, but a normal result does not exclude depleted cell and bone magnesium. Urine magnesium shows kidney excretion and helps identify whether magnesium is lost in urine or deficiency has another source, such as the intestines. Magnesium deficiency often accompanies low potassium or calcium and makes their replacement difficult.

Panels including this test

Test Magnesium (serum or urine) is also included in this panel.

Energy balance panelvitamin D, vitamin B12, folic acid, calcium, magnesium and 498.50€Details ›

All panels including Magnesium (serum or urine) (7) ›

Preparation and sample

Venous blood is used for blood testing. Fasting is unnecessary unless other fasting tests are performed the same day. Tell your doctor and collection staff which medicines you take and whether you take magnesium or other supplements.

For urine testing, a doctor will say whether a single sample is sufficient or 24-hour urine is needed. Every branch accepts urine samples.

How to interpret the result

Magnesium is expressed in mmol/L. The adult serum reference interval is usually approximately 0,65 to 1,05 mmol/L, but depends on age and method; the interval printed on your report applies. Urine magnesium is interpreted alongside urine volume and collection duration. A doctor assesses results alongside symptoms, medicines and other electrolytes.

Low magnesium

The most common causes are gastrointestinal loss (persistent diarrhoea, poor absorption), excessive alcohol consumption, poor diet and medicines: diuretics, long-term proton pump inhibitors and some hospital medicines. Magnesium is also lost in urine with poorly controlled diabetes.

Mild deficiency often causes no symptoms. More pronounced deficiency causes fatigue, nausea, weakness, muscle cramps and twitching, tremor and heart rhythm disturbances, while very low magnesium can cause seizures. An irregular heartbeat with faintness or whole-body convulsions requires urgent assessment.

When serum magnesium is low, urine magnesium helps distinguish kidney loss from intestinal loss. A doctor decides on replacement, formulation and dose according to the cause.

High magnesium

Raised magnesium is rare. It most often occurs with reduced kidney function, particularly when taking magnesium preparations such as some antacids, laxatives or supplements. With raised magnesium, a doctor therefore also checks creatinine. More marked elevations may cause weakness, nausea, low blood pressure and a slow heart rate.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed under Contact). Urine testing is also available without a referral, and every branch accepts urine samples. Prices for all tests and panels are in the service price list.

Frequently asked questions

Can blood magnesium be normal despite deficiency?

Yes. Only a small portion of body magnesium is in blood, and its level is maintained even at the expense of bone stores. With a normal result but clear symptoms or risk factors, a doctor therefore sometimes requests urine magnesium or assesses the situation using other findings.

Are leg cramps a sign of magnesium deficiency?

They can be, but more often are not. Calf cramps are common in healthy people: after exertion, particularly in heat, with insufficient fluids, in older age and later pregnancy, with some medicines and often without a clear cause. Magnesium, potassium and calcium results help avoid guessing.

Should I stop taking magnesium before testing?

Do not stop medicines or supplements yourself. If taking magnesium, agree with your doctor whether to omit it before testing, depending on what the result is meant to check, and mention this during collection.