5th April 2024

Investigations

As a small amount of total body magnesium is extracellular, serum magnesium may be normal in early mild deficiency and a more accurate picture can be obtained with ionised magnesium levels. The majority of extracellular magnesium is protein bound; therefore protein levels are important as protein loss can affect the reading. 

Calcium, phosphate and potassium levels should be checked as hypomagnesaemia is associated with hypokalemia,  hypocalcemia and hypophosphatemia. Due to the association with diabetes, glucose levels should be checked. 

The ECG in hypomagnesaemia may include broad QRS complexes, Qt interval and PR prolongation.  

To determine the cause of low magnesium, 24-hour urinary magnesium excretion is sometimes used, with high urinary levels indicating renal wasting. Fractional excretion of magnesium provides a ratio which is useful to determine if the cause is renal or extra-renal.