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Test Code MAGNR Magnesium, Random, Urine

Useful For

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.

Method Name

Colorimetric Endpoint Assay

Reporting Name

Magnesium, Random, U

Specimen Type

Urine


Specimen Required


Collection Container/Tube: Plastic urine container

Submission Container/Tube: Plastic, 5-mL tube (T465) or a clean, plastic aliquot container with no metal cap or glued insert

Specimen Volume: 4 mL

Collection Instructions:

1. Collect a random urine specimen.

2. No preservative.


Specimen Minimum Volume

1 mL

Specimen Stability Information

Specimen Type Temperature Time
Urine Refrigerated (preferred) 14 days
  Frozen  14 days
  Ambient  72 hours

Reject Due To

Hemolysis

NA

Lipemia

NA

Icterus

NA

Other

NA

Clinical Information

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 is involved in intermediary metabolic processes; about 70% is present in free form, while the other 30% is 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

Random Magnesium/Creatinine Ratio: ≥0.035 mg/mg

 

Reference values have not been established for patients <18 years and >83 years of age.

Interpretation

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

In the presence of hypomagnesemia, a 24-hour urine magnesium >24 mg/day or fractional excretion >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

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

Method Description

In alkaline solution, magnesium forms a purple complex with xylidyl blue, diazonium salt. The magnesium concentration is measured photometrically via the decrease in xylidyl blue absorbance.(Package insert: Roche MG2 kit, Indianapolis, IN, V2 2012)

Day(s) and Time(s) Performed

Monday through Sunday; Continuously.

Analytic Time

Same day/1 day

Specimen Retention Time

7 days

Performing Laboratory

Mayo Medical Laboratories in Rochester

Test Classification

This test has been modified from the manufacturer’s instructions. Its performance characteristics were determined by Mayo Clinic in a manner consistent with CLIA requirements.

CPT Code Information

83735

LOINC Code Information

Test ID Test Order Name Order LOINC Value
MAGNR Magnesium, Random, U In Process

 

Result ID Test Result Name Result LOINC Value
MGCO Magnesium, Random, U In Process
CREA7 Creatinine Concentration 2161-8
MCTR Magnesium/Creatinine Ratio In Process