A number of studies have now concluded that serum magnesium tests are not an accurate indicator of total body magnesium stores. As such caution should be used, when relying on serum testing to exclude magnesium deficiency.
Numerous studies have shown that magnesium deficiency is common and may be present in over 10% of hospitalized patients, as well as in the general population.
In 183 peer-reviewed studies published from 1990 to 2008, magnesium deficiency was associated with increased prevalence and risk in 11 major conditions. 
Many physicians are still under the impression, that serum magnesium tests are the gold standard for evaluating magnesium status.
However, current scientific research has found the complete opposite to be true, which is that serum magnesium tests are an extremely poor indicator of total body magnesium stores.
Serum Magnesium Only Accounts For Around 1% Of Total Body Magnesium Stores
A study from 2003 on magnesium metabolism and it’s disorders stated that the correlation between serum total magnesium and total body magnesium status is poor. 
As a result, it’s not difficult to see why serum magnesium tests are a poor choice for determining true magnesium status and a high rate of magnesium deficiency is probably going undetected in adults. Serum levels of magnesium also tend to be maintained, at the expense of other cells in the body.
One study from 2005 evaluating magnesium status and its role in hypertension also stated that “serum magnesium does not reflect true magnesium status as do intracellular magnesium measurements.”. 
Red Blood Cell Magnesium Testing
A more accurate test for checking magnesium status is thought to be the red blood cell magnesium test or an RBC magnesium test as its known.
The red blood cell magnesium test checks the level of magnesium inside the red blood cells, which is thought to correlate better with intracellular levels.
An underestimated problem…
A fairly recent study from 2010 concluded that it is an underestimated problem of physicians using serum magnesium measurements to exclude magnesium deficiency in adults and that a health warning is required for “normal” results.
The study found that an important cause for under diagnosis of magnesium deficiency is that serum magnesium, the most commonly used test, can be normal despite negative body stores.
The study concluded:
The perception that “normal” serum magnesium excludes deficiency is common among clinicians.
This perception is probably enforced by the common laboratory practice of highlighting only abnormal results.
A health warning is therefore warranted regarding potential misuse of “normal” serum magnesium because restoration of magnesium stores in deficient patients is simple, tolerable, inexpensive and can be clinically beneficial. 
 The underestimated problem of using serum magnesium measurements to exclude magnesium deficiency in adults; a health warning is needed for “normal” results.
 Magnesium Metabolism and its Disorders
 [Magnesium and hypertension].
The information in this article has not been evaluated by the FDA and should not be used to diagnose, cure or treat any disease, implied or otherwise.