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- 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.