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Mg Dr. Hamed Khouja
Magnesium
Physiologic Role & Function
Factors influence Mg blood concentration
Clinical significant
Method of determination
Physiologic Role & Function
Second Most abundant intracellular cation after K
~ 65% of Mg is in the bone and teeth (structural role)
~ 34% intracellular fluids (co-factor for enzymes and essential for cell functions)
~ 1% extracellular fluids
Mg in plasma:
o Makes approximately 1% of the whole body Mg which is divided into;
22-27% is bound to: Albumin
8% is bound to other globulins
1-5% is complexed with citrate, phosphate, oxalate, carbonate
65-70% is free form (ionized) (Mg 2+)
Mg is an important co-factor in reactions that require ATP as a source of
energy
It is an essential cation for many biological systems in the body
Carbohydrate metabolism
Nerve conduction
Neuromuscular contraction
Blood coagulation
It is a co-factor for over 300 enzymes in the body such as Na, K-ATPase, CK,
ALP, ACP
It is essential for maintaining the macrormolecular structure of RNA, DNA
and Ribosomes Its concentration in RBCs is 10 times than in plasma
Dietary sources for Mg include; whole grain cereals, legumes, milk, nuts,
bananas, leafy vegetables
Factors Regulating the concentration of Mg.
Regulation of circulating levels of Mg is not well understood
Effects of pH:
Decrease pH increase Mg2+ and decrease bound Mg total Mg not changed
Increase pH decrease Mg2+ and increase bound Mg total Mg not changed
Effects of Protein
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Mg Dr. Hamed Khouja
-affects bound and tMg , does not affect Mg2+
- hyperproteinaemia-increased bound Mg, increased tMg no change in Mg2+
- hypoproteinaemia- decreased bound Mg, decreased tMg no change in Mg2+
PTH Increase tubular reabsorption of Mg
Thyroid hormones, calcitonin, GH, hyperaldosteronism Decrease renal reabsorption of MgIncrease Na+, Increase Ca. Alcohol ingestion Decrease renal reabsorption of Mg
Clinical significance
A) Hypomagnesaemia
Associated with: - Chronic diarrhea -malabsorption - hepatic cirrhosis -Pancratitis
-ulcerative colitis -Hemodialysis - - alcoholism -osmotic diuresis
-diuretic drugs -hypercalcemia -infants on high Pi drink -Insulin administration
-hyperaldosteronism -hypoparathyroidism -hyperthyroidism
Symptoms
Similar to hypocalcaemia and includes; weakness, sever neuromuscular irritability (tetany)
- hyperfiexia -Convulsion - disorientation -cardiac arrthymias
B) Hypermagnesaemia
Associated with: -Oliguria - dehydration -Addisons disease
- diabetic acidosis -Chronic renal filer -hypothyroidism
-ingestion of Mg containing gastroenteric medications (Milk of Magnesia)
Symptoms
-Depressed neuromuscular activity (sedation) -Depressed. cardiac activity
-Nausea -Drowsiness -Hypotension -Respiratory depression
-Cardiac arrest -death
Determination of Mg
Sample: fasting serum. Li heparin plasma (separate immediately after collection)
Do not use EDTA, Citrate or Oxalate tubes
Avoid hemolysis (Increase in Mg)
Avoid prolonged tourniquet application
Separate serum as soon as possible to avoid haemolysis and leaksge of Mg from
RBCs
Urine, 24h acidified with 6M HCl
Avoid metal containing container
Methods
1) Atomic absorption spectrophotormetry (AAS) [reference method]
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Mg Dr. Hamed Khouja
Principle:
Same as in Ca determination -Sensitive, specific, easy to apply -Wave length 285.2 nm
Interference:1) Phosphate: Use Lanthanum chloride solution to remove Pi (1/50 dil of the sample
and control with lanthanum chloride solution)
2) Protein: deproteinize serum or plasma with trichloroacetic acid, centrifuge and use
the supernatant.
Urine by AAS. dilute 1/50 Lanthanum chloride solution for urine.
2) Colorimetric Methods
Mg forms coloured complexes with some dyes such as; Titan Yellow dye, Methylthymol
blue (MTB), Calmagite, Magon, Erichrome, Black T.
3) Complexometric method
8-hydroxy quinoline has high affinity for Mg
-EDTA titration
Major Precautions with methods 2&3:
In both the colorimetric and complexometric procedures,
o Ca is the major interfering substance as it also react with the Mg agents
o Ba-EGTA is used in these procedure to preferentially chelate Ca and prevents
its interference.
The linearity of the measurement is checked with a range of Mg standards because
the reaction obeys Beers Law only on a short range of Mg concentration.
AAS method for Mg is preferred to other methods because it offers better accuracy
and precision.
Normal Range by AAS
Adult serum 0.65-1.05 mmol/L
Urine 3-5 mmol/day
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