Total Calcium needs to be interpreted in the context of serum albumin levels
WHY? Calcium is protein bound (primarily to Albumin). Hence in states of low or high albumin, the total Calcium level may not reflect the amount of ionized (free) Ca2+; which is the form that is physiologically active.
There are two formulas:
SI: Calcium (corrected) = Serum Calcium + 0.025 x (Normal albumin – Patient albumin) [sometimes the 0.025 will be approximated as 0.02]
*Normal albumin is typically 40g/L or 44g/L for most hospitals (US: 4g/dL or 4.4 g/dL)
US: Calcium (corrected) = Serum Calcium + 0.8 x (Normal albumin – Patient albumin)
Rule of Thumb: For every drop in 10g/L of Albumin, your total Calcium will increase by 0.2mmol/L (or 0.8 mg/dL)
NOTE: Evidence exists that the Payne correction formula does not correlate well with Ionized-Ca2+.
References
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