Ionized calcium: serum levels during symptomatic hypocalcemia.

The concentration (activity) of ionized calcium in serum [Ca-++] was measured in nine patients with symptomatic hypocalcemia of diverse etiology. In all nine patients the [Ca-++] was smaller than or equal to 2.50 mg/dl. No significant correlation was found between the [Ca-++] and serum levels of either total calcium or phosphate. Use of the McLean-Hastings nomogram failed to reveal any significant statistical correlation between the predicted [Ca-++] values and those actually measured. These data indicate that: (1) symptomatic hypocalcemia occurs when levels of ionized calcium in serum fall near or below a critical threshold concentration of 2.50 mg/dl and (2) the [Ca-++] must be measured directly to confirm the clinical impression of hypocalcemia.