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Test Catalog

Test ID: MAGRU    
Magnesium/Creatinine Ratio, Random, Urine

Useful For Suggests clinical disorders or settings where the test may be helpful

Assessing the cause of abnormal serum magnesium concentrations

 

Determining whether nutritional magnesium loads are adequate

 

Calculating urinary calcium oxalate and calcium phosphate supersaturation and assessing kidney stone risk.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Magnesium, along with potassium, is a major intracellular cation. Magnesium is a cofactor of many enzyme systems. All adenosine triphosphate-dependent enzymatic reactions require magnesium as a cofactor. Approximately 70% of magnesium ions are stored in bone. The remainder are involved in intermediary metabolic processes; about 70% are present in free form, while the other 30% are bound to proteins (especially albumin), citrates, phosphate, and other complex formers. The serum magnesium level is kept constant within very narrow limits.

 

Renal handling of magnesium is determined by the combination of filtration and reabsorption. Roughly 70% of the magnesium in plasma is filtered by the glomeruli; 20% to 30% of the filtered magnesium is reabsorbed in the proximal tubule, while less than 5% is reabsorbed in the distal tubule and collecting duct.(1)

 

Numerous causes of renal magnesium wasting have been identified including (but not limited to) congenital defects (including Barter and Gitelman syndrome), various endocrine disorders (including hyperaldosteronism and hyperparathyroidism), exposure to certain drugs (ie, diuretics, cis-platinum, aminoglycoside antibiotics, calcineurin inhibitors), and other miscellaneous causes (including chronic alcohol abuse). Gastrointestinal conditions associated with fat malabsorption and chronic diarrhea can cause fecal magnesium loss and hypomagnesemia.

 

High levels of plasma magnesium are typically only seen in patients with decreased renal function, after administration of a magnesium load large enough to exceed the kidneys' ability to excrete it, or a combination of the two.(2)

 

Magnesium is an inhibitor of calcium crystal growth and contributes to urinary calcium oxalate and calcium phosphate supersaturation. However, low urinary magnesium in isolation has not been identified as a common cause of kidney stones, nor has magnesium supplementation been proven as an effective therapy for stone prevention.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

1 month-<12 months: 0.10-0.48 mg/mg creat

12 months-<24 months: 0.09-0.37 mg/mg creat

24 months-<3 years: 0.07-0.34 mg/mg creat

3 years-<5 years: 0.07-0.29 mg/mg creat

5 years-<7 years: 0.06-0.21 mg/mg creat

7 years-<10 years: 0.05-0.18 mg/mg creat

10 years-<14 years: 0.05-0.15 mg/mg creat

14 years-<18 years: 0.05-0.13 mg/mg creat

18 years-83 years: 0.04-0.12 mg/mg creat

Reference values have not been established for patients who are less than 1 month of age.

Reference values have not been established for patients who are greater than 83 years of age.

Interpretation Provides information to assist in interpretation of the test results

Urinary magnesium excretion should be interpreted in concert with serum concentrations.

 

In the presence of hypomagnesemia, a 24-hour urine magnesium above 24 mg/day or fractional excretion above 0.5% suggests renal magnesium wasting. Lower values suggest inadequate magnesium intake and/or gastrointestinal losses.

 

In the presence of hypermagnesemia, urinary magnesium levels provide an indication of current magnesium intake.

 

Lower urinary magnesium excretion increases urinary calcium oxalate and calcium phosphate supersaturation and could contribute to kidney stone risk.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Urinary magnesium excretion must be interpreted with caution during periods of intravenous magnesium infusion.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Delaney MP, Lamb EJ: Kidney disease. In: Rifai N, Horvath AR, Wittwer CT, eds: Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018:1309

2. Al Ghamdi SM: Magnesium deficiency: pathophysiologic and clinical overview. Am J Kidney Dis. 1994;24(5):737-752

3. Sutton RA: Abnormal renal magnesium handling. Miner Electrolyte Metab. 1993;19(4-5):232-240