Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (762 page)

   Falsely elevated values may occur in serum from patients who have developed human antimouse antibodies or heterophilic antibodies.

Suggested Readings

Basuyau JP, Mallet E, Leroy M, et al. Reference intervals for serum calcitonin in men, women, and children.
Clin Chem.
2004;50:1828–1830.

Saad MF, Ordonez NG, Rashid RK, et al. Medullary carcinoma of the thyroid. A study of the clinical features and prognostic factors in 161 patients.
Medicine (Baltimore).
1984;63: 319–342.

CALCIUM, IONIZED

   Definition

   Ionized calcium is the physiologically active form of calcium. Ionized calcium homeostasis is regulated by the parathyroid glands, bone, kidney, and intestine. It is most frequently used in ICUs and operating rooms.

   
Normal range:
4.6–5.3 mg/dL.

   
Critical range:
<4.1 or >5.9 mg/dL.

   Use

   In patients with hypocalcemia or hypercalcemia with borderline serum calcium and altered serum proteins.

   Approximately 50% of calcium is ionized; 40–45% is bound to albumin; 5–10% is bound to other anions (e.g., sulfate, phosphate, lactate, and citrate); only the ionized fraction is physiologically active. Total calcium values may be deceiving, because they may be unchanged even if ionized calcium values are changed (e.g., increased blood pH increases protein-bound calcium and decreases ionized calcium, and PTH has the opposite effect) (blood pH should always be performed with ionized calcium, which is increased in acidosis and decreased in alkalosis). However, in critically ill patients, elevated total serum calcium usually indicates ionized hypercalcemia, and normal total serum calcium is evidence against ionized hypocalcemia.

   Ionized calcium is the preferred measurement rather than total calcium, because it is physiologically active and can be rapidly measured, which may be essential in certain situations (e.g., liver transplantation and rapid or large transfusion of citrated blood make interpretation of total calcium nearly impossible).

   Life-threatening complications are frequent when serum ionized calcium <2 mg/dL.

   With multiple blood transfusions, ionized calcium <3 mg/dL may be an indication to administer calcium.

   Interpretation