This patient's magnesium level should be checked. He has a very low calcium level that is likely contributing to his clinical findings of tremulousness, muscle irritability, and electrocardiogram changes. He struggles with alcohol abuse, and his low albumin level suggests malnutrition, likely due to his chronic alcohol intake as a primary source of calories. Magnesium deficiency is common in persons who abuse alcohol. Furthermore, he has had diarrhea for several days, which will also deplete his magnesium stores. This patient's low calcium level should promote parathyroid hormone (PTH) secretion to help correct the hypocalcemia. However, decreased levels of magnesium impair release of PTH; levels in patients with significant magnesium deficiency are either low or inappropriately in the normal range. Low magnesium levels are also associated with resistance to PTH activity at the level of bone, further contributing to hypocalcemia. Therefore, in patients with hypocalcemia and hypomagnesemia, it is crucial to correct the magnesium level (to at least 2 mg/dL [0.83 mmol/L]), as it is difficult to increase calcium levels until this is done.

1,25-Dihydroxyvitamin D has a short half-life, and its measurement only reflects the active levels of vitamin D. Since active levels tend to vacillate frequently based on immediate need, this level would not be diagnostically helpful. 25-Hydroxyvitamin D levels have a longer half-life and, therefore, more accurately reflect the total body stores of vitamin D.

Measuring 24-hour urine calcium excretion in this patient would not be diagnostically helpful as significant urinary loss of calcium is uncommon, and the urine calcium level in this patient would be expected to be low due to compensatory kidney retention of calcium.

Checking the ionized calcium level is indicated in settings where abnormal serum protein binding of calcium is possible. This test is not needed in this symptomatic patient who has no suggestion of excessive protein binding of calcium.