Page 231 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Disorders of Magnesium: Magnesium Deficit and Excess 221

DIAGNOSIS OF MAGNESIUM rapidity of loss of magnesium from the patient. Patients
DEFICIT that lose magnesium rapidly will tend to draw heavily
from the serum magnesium to replace an acute intracel-
The diagnosis of a magnesium deficit continues to be lular need and may be more likely to have low total serum
controversial. The fact that 99% of the body’s magnesium or ionized serum magnesium levels. Chronic mild inade-
stores are located within cells presents a diagnostic chal- quate dietary intake of magnesium may allow sufficient
lenge for clinicians hoping to identify depletion of the time for compensatory mechanisms to increase gastroin-
body’s magnesium. Given our currently limited ability testinal absorption, renal reabsorption, and possibly skel-
to peer inside of cells on a routine basis clinically, it should etal liberation of sufficient magnesium to maintain
51,54
not be surprising that the diagnosis of magnesium deficit normal serum and total body magnesium. When
is difficult and controversial. Despite the challenges, these compensatory mechanisms are active, they may be
however, numerous diagnostic methods have emerged much more effective in coping with an additional acute
in concert with the renewed clinical interest in magne- loss, and allowing normal serum levels to be maintained.
sium over the past 20 years. These efforts can be broadly Based on these alterations between serum and ionized
divided into two separate categories: methods that assess fractions, one study suggested the use of a ratio between
magnesium (both ionized and total) in various tissues total serum and ionized magnesium as being more help-
(including blood) and methods that assess magnesium ful. 51 Concurrent hormone activity, albumin concentra-
handling physiology. tion, sample handling, and acid base status of the
The challenge of choosing a tissue to sample from to patient may all play roles in serum magnesium concentra-
{
detect a magnesium deficit is to choose one that is most tion (Chew, DJ, unpublished data). In addition, redistri-
often reflective of a true total body deficit of magnesium. bution of magnesium from the serum compartment has
Total serum magnesium is the most commonly employed been reported to occur in acute pancreatitis and
method of assessing magnesium status because of the ease myocardial infarction of humans and thus could also
of obtaining serum samples from patients and the relative affect the serum magnesium status and add further diffi-
simplicity of and the ability to automate the assay. More culty in interpretation of serum magnesium levels. Several
recently the development of technology that allowed studies have also called into question the ion-selective
measurement of ionized serum magnesium has emerged probe technology that has been used to measure ionized
28,36
and is becoming widely available. There is no question serum magnesium. In combination, these factors add
that blood forms the main method of magnesium trans- a large degree of uncertainty to the interpretation of
port from dietary ingestion, urinary retention, and move- blood magnesium levels.
ment of magnesium between intracellular stores. Cellular Measurement of magnesium in red blood cells, white
intake of magnesium occurs when ionized magnesium blood cells, and muscle tissue have also been investigated
crosses from the blood through the cell membrane and as potential assays that are more reflective of intracellular
17,54,140,150
then is complexed and harnessed into the intracellular magnesium stores. Due to the complexity of
magnesium dependent activities. Ionized magnesium the assays, nonehas found common clinicalusage.In addi-
appears to equilibrate rapidly across the cell membrane, tion, results has not consistently correlated with clinical
thus extracellular ionized magnesium may be reflective assessment of magnesium deficit. 140,150 Newer
of intracellular stores. The larger question, however, is technologies that may be able to assess intracellular ion-
how reflective of a total body magnesium deficit is a blood ized magnesium concentrations, such as nuclear magnetic
sample? Total serum magnesium represents 1% of the resonance spectroscopy and fluorescent intracellular
body’s magnesium stores, and ionized serum magnesium probes, hold much promise because they are non-invasive
represents 0.2% to 0.3% of the total body magnesium and they assess intracellular magnesium stores. 94,173 Such
stores. The lack of a gold standard test with which to technologyhasnotyetfoundwidespreadclinicalusagebut
compare both total serum magnesium and ionized serum is an important research technology.
magnesium assays contributes to the confusion regarding Assessment of physiologic magnesium handling has
diagnosis of a magnesium deficit. While it is attractive been evaluated in one of two ways: assessment of renal
because of its simplicity, serum magnesium does not cor- magnesium handling and testing magnesium retention.
relate with the diagnosis of a suspected magnesium deficit Both are based on the concept that active renal retention
based on clinical signs, nor does it appear to correlate well of magnesium during total magnesium deficit should
with serum ionized magnesium.* occur. In addition, they assume that renal function and
There may be several factors to consider when renal magnesium handling are adequate and appropriate
interpreting the results of a blood magnesium sample, to the patient’s current status. These assays cannot be
such as adequate dietary intake of magnesium and the used in patients with inadequate renal function or in

*References 54, 75, 84, 110, 126, 189. *References 51, 75, 84, 95, 168, 184.