Laboratory Data

1. Serum electrolytes. In addition to total calcium, focus on potassium, phosphate, and magnesium levels. The latter two are not usually included in standard panels and may have to be ordered separately. Serum calcium should be interpreted in relation to serum albumin (see II, B, earlier). Hyperkalemia may be a sign of tumor lysis. Serum phosphate is elevated in renal failure, tumor lysis, rhabdomyolysis, phosphate enemas, and parathyroid disorders. It is also seen in most of the neonatal hypocalcemic disorders. Hypophosphatemia is a sign of vitamin D disorders, hungry bone syndrome, and Fanconi syndrome. Severe hypomagnesemia, < 1 mg/dL, is a cause of refractory hypocalcemia.

2. Serum albumin. As previously described.

3. Ionized calcium. Particularly valuable in the presence of alkalosis and chelators, which may selectively lower ionized calcium. In confusing cases, ionized calcium can help with diagnosis and management.

4. BUN and creatinine. Signs of renal failure, acute or chronic.

5. PTH level. Should be interpreted in relation to serum calcium level.

6. Vitamin D levels. 25-Hydroxyvitamin D identifies deficiency or abnormalities of metabolism whereas 1,25-dihydroxyvitamin D may be helpful in patients with vitamin D-dependent states and renal disease.

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