Tinnitus and blood testing

Yeah, magnesium measurement doesn't seem to be very reliable.

Measuring total serum magnesium is a feasible and affordable way to monitor changes in magnesium status, although it does not necessarily reflect total body magnesium content.

The most common test for the evaluation of magnesium levels and magnesium status in patients is serum magnesium concentration [21, 56], which is valuable in clinical medicine, especially for rapid assessment of acute changes in magnesium status [17]. However, serum magnesium concentration does not correlate with tissue pools, with the exception of interstitial fluid and bone. It also does not reflect total body magnesium levels [17, 57]. Only 1% of total body magnesium is present in extracellular fluids, and only 0.3% of total body magnesium is found in serum, and so serum magnesium concentrations [22] are poor predictors of intracellular/total body magnesium content [7]. This situation is comparable to assessing total body calcium by measuring serum calcium, which, too, does not adequately represent total body content. As with many reference values, laboratory parameters will also vary from laboratory to laboratory resulting in slightly varying ranges for the ‘healthy’ populations evaluated. What is considered the ‘normal level’ might actually be slightly too low, representing a mild magnesium deficit present in the normal population [17].

There are also the 24-hour urine excretion test, and the retention test.
The full article from the below link:
http://ckj.oxfordjournals.org/content/5/Suppl_1/i3.full