Magnesium XL FS is a photometric test to determine magnesium in serum, plasma, cerebrospinal fluid or urine. Magnesium deficiency is a quite common disorder, which might result from malnutrition, malabsorption, renal loss and endocrinological disturbances and is associated with complications like neuromuscular irritability (e.g. tremor, seizures) and cardiac symptoms (e.g. tachycardia, arrhythmia).