magnesium (Mg+2) in 24 hour urine

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Indications

Reference interval

Principle

About 40 per cent of the average adult daily dietary intake of magnesium (300 mg) is absorbed in the small intestine & excreted in the urine.

The absorption process appears to be poorly controlled, & homeostasis is maintained largely by renal excretion, which is regulated by tubular reabsorption.

The ACA magnesium method is a modification of the methylthymol blue (MTB) complexometric procedure.

MTB forms a blue complex with magnesium.

Calcium ilnterference is minimized by forming a complex between calcium & Ba-EGTA (chelating agent).

The amount of MG-MTB complex formed is proportional to the magnesium concentration & is measured using a two-filter (600-510 nm) end-point technique.

Clinical significance

Magnesium depletion is clinically more significant & frequent than an excess, with a prevalence of 11 per cent in hospitalized patients.

Signs & symptoms of magnesium depletion do not usually appear until extracellular levels have fallen to 1 MEQ/L or less.

Manifestations of significant magnesium depletion include weakness, muscle fasciculations, depression, agitation, seizures, hypocalcemia, hypokalemia, & cardiac arrhythmias.

Cause for symptomatic hypomagnesemia include malabsorption, severe diarrhea, nasogastric suction with administration of magnesium-free parenteral fluids, alcoholism, acute pancreatitis, early chronic renal disease, malnutrition, excessive lactation, chronic dialysis, digitalis intoxication, hyperparathyroidism, hypoparathyroidism, hyperaldosteronism, diabetes mellitus, diuretic therapy & porphyria with inappropriate secretion of antidiuretic hormone.

Increases

Decreases

Specimen

2 mL aliquot of a 24-hour urine collection.

Proper 24 hour urine collection procedureshould be followed, & collection container should be refrigerated at 2-6 C during collection.

Upon receipt in the work area, it should be well-mixed & measured in a graduated cylinder.

The total volume should be recorded.

Acidify patient samples with 1 part concentrated HCL to 100 parts of urine prior to analysis.

Centrifuge if a precipitate forms.

More general terms

Additional terms

References

  1. Kaplan, Lawrence A. & Pesce, Amadeo J., Clinical Chemistry: Theory, Analysis, & Correlation, 2nd Edition, The C.V. Mosby Company, St. Louis, MO, 1989, pp. 875-879.
  2. Henry, John Bernard, M.D., Clinical Diagnosis & Management by Laboratory Methods, 18th Edition, W.B. Saunders Company, Philadelphia, PA, 1991, pp. 165-166.
  3. ACA IV Discrete Clinical Chemistry Analyzer Instrument Manual, Volume 3A, Chapter 6: Test Methodology, MG 8.
  4. Clinical Guide to Laboratory Tests, 4th edition, HB Wu ed, WB Saunders, Philadelphia, 2006
  5. Panel of 8 tests Laboratory Test Directory ARUP: http://www.aruplab.com/guides/ug/tests/0020477.jsp
  6. Panel of 29 tests Laboratory Test Directory ARUP: http://www.aruplab.com/guides/ug/tests/0020805.jsp