Magnesium is a cation (positively charged ion) primarily present in cells where it plays a role in several metabolic reactions. About 70% of the body’s magnesium is found in the bones, and blood levels are maintained within very narrow limits. The kidneys are responsible for maintaining blood magnesium levels within very narrow limits. Several medication- or endocrine-induced kidney impairments can result in the loss of magnesium by the kidneys. In contrast, kidney failure (uremia) is a cause of magnesium accumulation in the blood. In the kidney, magnesium inhibits the formation of stones. Low urine magnesium levels can therefore contribute to the formation of oxalate or calcium phosphate stones. Results are reported in millimoles of magnesium per day (mmol/d) for 24-hour sample collections and in millimoles of magnesium per millimole of creatinine for isolated mictions. Urine magnesium levels must be interpreted along with blood magnesium levels.
If blood magnesium is low, urine levels greater than 1 mmol/d or 0.16 mmol/mmol creatinine on an isolated miction indicate a probable loss of magnesium by the kidneys. Levels below 1 mmol/L or ≤ 0.16 mmol/mmol creatinine suggest inadequate dietary intake or intestinal losses. If blood magnesium is high, urine magnesium helps determine dietary intake of magnesium.