Vitamins, minerals and amino acids
Magnesium
Magnesium in urine shows how much magnesium the kidneys excrete; it is not a measure of the magnesium store.
Everything about this testWhat it measures
The analysis measures how much magnesium is excreted in the urine. Magnesium is in every cell of the body and is needed for energy production, the working of muscles and nerves and the strength of bones. About half of the store is in the bones and only about 1 percent in the blood. The body keeps the amount of magnesium constant by adjusting absorption in the intestine and excretion by the kidneys; the urine result mainly reflects that regulation and the intake of the past day.
- Loss of magnesium through the kidneys as the explanation for a confirmed low magnesium in the blood
- An impression of magnesium intake from food and supplements in the days before the measurement
High
A high excretion usually means that you take in a lot of magnesium, for example through supplements or laxatives containing magnesium; the kidneys simply remove the excess. In someone with a confirmed low magnesium in the blood, a high excretion instead fits loss through the kidneys, for example due to water pills (diuretics), poorly controlled diabetes or certain medicines. The result can only be interpreted well in that combination.
Low
A low excretion fits a low intake or kidneys that are holding on to magnesium. In someone with a low magnesium in the blood, a low excretion points more to too little absorption through the intestine, for example with prolonged diarrhea, a bowel disease or alcohol use. On its own, a low urine value says nothing about symptoms and is not proof of a magnesium deficiency.
If your result is abnormal
With symptoms, the GP first measures magnesium, calcium and potassium in the blood. If the magnesium in the blood is low, the urine excretion can help find the cause; if needed, referral to an internist (specialist in internal medicine) follows.
Reliability
Magnesium in urine is mainly informative when a low magnesium has been found in the blood: the result then helps to distinguish between too little absorption and too much loss through the kidneys. In people without symptoms and with a normal magnesium in the blood, a single value says little. A single urine sample varies strongly with how much you drink and with what you ate the day before. That is why the result is usually corrected for the creatinine content, or urine is collected over 24 hours.
Limitations
- A single sample depends strongly on fluid intake and on what you ate or swallowed the day before.
- The result can only be interpreted together with the magnesium in the blood; separately they say little.
- Water pills (diuretics), stomach acid inhibitors, laxatives containing magnesium and supplements change the excretion.
- There are no agreed cutoff values; the laboratory decides which limit it uses.
How common is this
The average magnesium intake in the Netherlands is sufficient; a deficiency from diet alone is uncommon. A doctor mainly sees a low magnesium in the blood with prolonged diarrhea, alcohol use, malnutrition, some bowel diseases and with use of water pills or stomach acid inhibitors.
Who is this useful for
Useful as a next step in someone with a confirmed low magnesium in the blood. In people without symptoms and without an abnormal magnesium in the blood, the urine test adds nothing. A result is not proof for or against a magnesium deficiency and is no reason to start taking supplements.
Downsides and risks
- A low result can wrongly be seen as proof of a magnesium deficiency, after which supplements are used unnecessarily.
- A high result due to supplement use can be confused with a kidney problem.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results