Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (765 page)

   Various formulas are available for calculating ionized calcium using total calcium, albumin, and total protein. However, these formulas may not apply in some situations; their use is discouraged.

   Hypomagnesemia or hypermagnesemia; patients respond to serum magnesium that becomes normal but not to calcium therapy. Serum magnesium should always be measured in any patient with hypocalcemia.

   Increase of ions to which calcium is bound:

   Phosphate (e.g., phosphorus administration in treatment of DKA, chemotherapy causing tumor lysis syndrome, rhabdomyolysis)

   Bicarbonate

   Citrate (e.g., during blood transfusion)

   Radiographic contrast media containing calcium chelators

CALCIUM, TOTAL

   Definition

   Ninety-nine percent of the body’s calcium is in bone. Of the remainder (of 1%) in blood, about 50% is ionized (free), about 10% is bound to anions (e.g., phosphate, bicarbonate), and about 40% (of 1%) in blood is bound to plasma proteins, (80–40%) of that to albumin.

   
Normal range:
8.7–10.7 mg/dL.

   
Critical values:
<6.6 or >12.9 mg/dL.

   Use

   Diagnosis and monitoring of a wide range of disorders, including disorders of protein and vitamin D, and diseases of the bone, kidney, parathyroid gland, or GI tract.