At present, there is no simple, rapid and accurate laboratory test to indicate the total body magnesium status. The most commonly used method for assessing magnesium status is the serum magnesium concentration. Other methods available for assessing magnesium status are listed in .
Tests used in assessing magnesium status
Serum total magnesium can be measured by a variety of techniques.4,17
Serum is preferable to plasma as the anticoagulant could be contaminated with magnesium or affect the assay. For instance, citrate binds not only calcium but also magnesium and affects fluorometric and colorimetric procedures. Haemolysis, bilirubin, lipaemia, high phosphate concentration and delay in separating serum can affect the measurement.18
In adults serum magnesium concentration is not influenced by sex or age except in the very elderly where it may be slightly higher.18
Serum magnesium concentration increases after a short period of maximal exercise but decreases after endurance exercises.18
It is lower during the third trimester of pregnancy and is higher in subjects on a vegetarian diet.18,19
Intra-individual variation in serum magnesium ranges between 3.4% and 4.7%.20
The total serum magnesium concentration is not the best method to evaluate magnesium status as changes in serum protein concentrations may affect the total concentration without necessarily affecting the ionised fraction or total body magnesium status. The correlation between serum total magnesium and total body magnesium status is poor.4,21
Measurement of ultrafiltrable magnesium may be more meaningful than the total magnesium as it is likely to reflect ionised magnesium concentration, but methods are not available for routine use.
In the last few years, ion selective electrodes for magnesium have been developed and several commercial analysers are now available for the measurement of ionised magnesium concentration.3,4
Measurement of ionised magnesium has been found to be useful in several clinical situations.3,4
However, results from different instruments do not agree as the electrodes are not entirely selective for ionised magnesium concentration and a correction is applied based on the ionised calcium concentration.4
Red cell magnesium concentration can be determined easily but does not seem to correlate well with total body magnesium status or with other measures of magnesium status.22
The magnesium content of mononuclear cells may be a better predictor of skeletal and cardiac muscle magnesium content. However, this method is technically more difficult and intraindividual variation is high at about 12–22%.23
As muscle contains nearly 30% of the total body magnesium it is an appropriate tissue for the assessment of magnesium status and studies in patients undergoing heart surgery showed that skeletal muscle magnesium was a better predictor of heart magnesium than lymphocyte or serum magnesium concentration.24
However, this is an invasive and expensive procedure requiring special expertise.
In the steady state, a 24-hour urine excretion of magnesium reflects intestinal absorption and is also of value in determining whether magnesium wasting is occurring by the renal route. In the presence of hypomagnesaemia, magnesium excretion > 1 mmol/day is suggestive of renal magnesium wasting. On the other hand, magnesium excretion < 0.5 mmol/day is suggestive of magnesium deficiency.10
The magnesium tolerance test has been used for many years and it appears to be an accurate means of assessing magnesium status. In this test, the percentage of magnesium retained after parenteral administration of magnesium is determined. The percentage of magnesium retained is increased in magnesium deficiency and is inversely correlated with the concentration of magnesium in bone.25
In a study of 23 healthy subjects, 13 hypomagnesaemic patients and 24 normomagnesaemic patients at high risk of magnesium deficiency, the percentage retention was 14±4% (mean ± SEM) in normals, 85±3% in hypomagnesaemic patients and 51±5% in patients at risk of developing magnesium deficiency. These data suggest that this test is a very sensitive method to detect magnesium deficiency.9,11
The test, however, depends on normal renal function and is of limited value in patients with renal magnesium loss.
Intracellular free magnesium concentration can be determined using fluorescent probes such as fura-2 or by nuclear magnetic resonance (NMR).4
Magnesium balance studies and studies using isotopes of magnesium are mainly used in research. Hair and tooth have also been used to assess magnesium status. Activation of enzymes such as creatine kinase and alkaline phosphatase by magnesium has also been examined as a measure of magnesium status. In experimental studies, however, it was not shown to be as good as serum or red cell magnesium concentration.26
In summary, no single method is satisfactory to assess magnesium status. The simplest, most useful and readily available tests are the measurement of serum total magnesium and the magnesium tolerance test.3,4
Ionised magnesium measurement may become more readily available with the development of reliable analysers.