- This test is done on the serum (3 to 5 mL clotted blood) of the patient.
- Fasting sample is preferred.
- Collect blood in the metal-free container.
- Separate the red cells immediately.
- Hemolysis interferes with the test. This may give false high value.
- Magnesium is Stable in blood for 3 hours and in serum at 2 to 8 °C for 8 hours.
- Separate the serum as soon as possible (within 45 minutes).
- Collect the sample in a prone position, because an upright position may increase magnesium by 4%.
- Avoid hemolysis because it gives false high value.
- There are drugs which give rise to low value are diuretics, insulin, and few antibiotics.
- There are a few drugs which give high values are antacids, calcium-containing medicines, laxatives, and thyroid medicines.
- Fasting sample is preferred.
- Avoid venous stasis.
Purpose of the test
- To evaluate the renal function.
- To evaluate the electrolytes.
- To evaluate magnesium metabolism to find deficiency or overload.
- The magnesium is the fourth most abundant cation in the body.
- Mg concentration in the cell is second to Potassium.
- Extracellular concentration is much lower.
- 30% Mg in plasma bound to albumin.
- 70% Mg exist as the ion Mg²+.
- Majority of the magnesium is intracellular.
- Magnesium is stored in:
- Bone 40 to 60%.
- Within the cells 30%.
- Muscles 20%.
- Serum 1% (RBC).
- Distribution of the magnesium in the body is as follows:
- Skeleton =55%
- Soft tissue = 45%
- Extracellular fluid = 1%
- Total weight in grams = 25 G
- Magnesium In the serum:
- 55% is free.
- 33% Associated with the protein (mainly albumin).
- 15% Complex with PO4, citrate and other anions.
- Magnesium intracellular and extracellular shown in the following diagram.
- Magnesium absorption and the source are:
- Green vegetables.
- It is absorbed in the upper intestine.
- 50% absorbed by the active transport or passive diffusion in jejunum and ileum.
- Vitamin D is not needed for the absorption.
- Majority of the Mg is not absorbed because of the formation of insoluble phosphates and soap in the gut.
- The average human being weighing 70 Kg will have 24 grams of magnesium.
- When the serum magnesium is low then it leads to increased neuromuscular excitability because magnesium competes and inhibit the entry of calcium into neurons.
- Functions of Magnesium:
- Magnesium cofactor for >300 enzymes in the body.
- Mg is important for the neuromuscular excitability.
- It has an important role for the smooth muscles contraction and relaxation.
- In cardiac patient low magnesium level increase the irritability of the cardiac muscles and may lead to arrhythmias.
- Mg had synergistic action along with sodium, calcium and potassium ions.
- Magnesium helps with the absorption of calcium from the intestine.
- Magnesium deficiency mobilizes calcium from the bones.
- Magnesium deficiency may lead to abnormal calcification in the aorta and kidneys.
- It is important for the clotting mechanism.
- It has an important role in:
- Oxidative phosphorylation.
- Cell replication.
- Protein Biosynthesis.
- Nucleotide metabolism (nucleic acid synthesis).
- Contraction of the muscular tissue.
- It is important for the action of ATP (adenosine triphosphate). It is bound to ATP molecule.
- It takes parts in phosphorylation which is the main source of energy.
- The kidney is the organ which the balance of Mg.
- Kidney will conserve when the intake is low and excrete the excess when the intake is high.
- In damaged kidney with decreased renal function, magnesium may be retained in the body and increase serum level of magnesium.
- Aldosterone promotes the excretion of the Mg++ together with K+ and the retention of Na+.
- Increased magnesium (Hypermagnesemia) is always due to increased intake of:
- Parenteral therapy in patients with renal failure.
- Administration of Mg in a patient with renal failure.
- This raised serum magnesium leads to:
- Decreased neuromuscular transmission (depression of neuromuscular system).
- Decrease the cardiac conduction.
- Slow deep tendon reflexes.
- There is respiratory depression.
- Magnesium-containing antacid may increase the magnesium level.
- Signs and symptoms of increased magnesium are:
- Slurred speech.
- Nausea and vomiting.
- Muscles weakness.
- Deep tendon reflexes disappear at the level of between 5 to 9 mg/dL.
- Depressed respiration is seen at 10 to 12 mg/dL.
- Cardiac arrest at a higher level.
- ECG shows typical changes of:
- Tall T wave.
- Widened QRS.
- Flattened P wave.
- Increased PR interval.
- Hypomagnesemia when there is abnormally low serum magnesium.
- Low magnesium is seen in:
- Intensive care units.
- Alcohol abuse increases the loss of magnesium in the urine.
- Sign and symptom of Low magnesium:
- Low magnesium level may lead to:
- Neuromuscular irritability with tetany and seizures.
- This is because Mg inhibits the entry of calcium into neurons.
- Cardiac arrhythmias may be seen. This may be related to hypokalemia and intracellular potassium depletion which occurs secondary to magnesium deficiency.
- Tachycardia and hypotension.
- Increased reflexes.
- Muscle cramps.
- ECG changes are difficult to document in low Mg. It is associated with other electrolyte imbalance like hypokalemia and hypocalcemia.
||1.5 to 2.2
|5 month to 6 year
||1.7 to 2.3
|6 to 12 year
||1.7 to 2.1
|12 to 20 year
||1.7 to 2.2
|60 to 90 year
||1.6 to 2.4
||1.7 to 2.3
|Higher in females during menses
- To convert into SI unit = 0.4114 = mmol/L
- Adults = 1.3 to 2.1 meq/L (1.8 to 2.6 mg/dL).
- Child = 1.4 to 1.7 meq/L (1.7 to 2.1 mg/dL).
- Newborn = 1.4 to 2.0 meq/L (1.5 to 2.2 mg/dL).
Increased magnesium (hypermagnesemia) >2.5 meq/L level is seen in:
- Oral intake of Mg in the presence of chronic renal failure.
- Renal failure and oliguria.
- Addison’s disease.
- Diabetes mellitus uncontrolled (rare).
- Antacids containing magnesium.
- Multiple myelomas.
- The enema containing Mg.
- Treatment of Mg deficiency.
Complication of Hypermagnesemia may cause respiratory paralysis and cardiac arrest.
Decreased Magnesium (Mg deficiency) <1.0 meq/L level is seen in:
- Malabsorption (Kwashiorkor).
- Malnutrition (a diet low in protein intake).
- Prolonged nasogastric suction.
- Extensive bowel resection.
- Acute and chronic diarrhea.
- Intestinal and biliary fistula.
- Neonatal primary hypomagnesemia.
- Alcoholism chronic.
- Chronic renal tubular disease.
- Chronic pyelonephritis.
- Interstitial nephritis.
- Renal tubular acidosis.
- Postrenal transplantation.
- Phosphate depletion.
- Hypercalcemia of any cause (Hyperparathyroidism).
- Chronic pancreatitis.
- Long Term I/V therapy,
- Diabetic acidosis.
- Idiopathic cause.
- Diuretic therapy (furosemide).
- Amphoterin B.
- Cardiac glycosides.
- Increased Excretion by the kidney:
- Tubular disorder.
- Excess lactation.
Complication of Hypomagnesemia leads to tetany, delirium, agitation, muscle weakness and ultimately cardiac arrhythmias.
There may be decreased Calcium, Phosphate and Potassium level.
Critical values are:
- Hypomagnesemia = < 1.2 mg/dL (< 0.5 meq/L) , there may be tetany.
- Hypermagnesemia = > 5 .0 mg/dL (> 3 meq/L).
Possible References Used
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