The magnesium test measures the level of magnesium in your blood. Magnesium is an essential mineral that plays a vital role in many bodily functions, including nerve and muscle function, bone health, heart rhythm regulation, and energy production. This test is usually ordered to check for magnesium deficiency (low levels) or excess (high levels), which can be caused by conditions such as kidney disease, malnutrition, chronic diarrhea, or certain medications.