• Total body calcium consists of about half biologically active (ionized) and half inactive (80% bound to albumin and 20% to other ions)
• hypocalcemia caused by hypoalbuminemia is physiologically insignificant, and correction factors are not accurate or reliable
• The best way to measure true active calcium is to order an ionized calcium level

There are several conditions that alter ionized calcium levels, including:

• alkalosis (increases binding to albumin)
• gas bubbles in the sample (false lowering of calcium)
• anticoagulants (must be collected in a red top tube)
• blood transfusions (binding to citrate)
• cardiopulmonary bypass
• drugs (aminoglycosides, cimetidine, heparin, theophylline)
• fat embolism
• hypomagnesemia (correcting mg levels may preclude need for Ca repletion)
• pancreatitis (several mechanisms, poor prognosis)
• renal insufficiency (impaired phosphate retention)
• sepsis

The bottom line is to measure ionized calcium, and consider all other factors that can be contributing to hypocalcemia in addition to repleting it.