Corrected Calcium for Albumin

Roughly 40 percent of the calcium in serum travels bound to albumin and is biologically inert; another 10 percent or so is complexed to anions, and only the remaining half is free ionised calcium doing anything. A standard chemistry panel reports total calcium, which means a patient with low albumin can look hypocalcaemic while their ionised calcium is perfectly normal. The correction adds back roughly 0.8 mg/dL of calcium for every 1 g/dL the albumin sits below 4.0. A measured calcium of 7.6 with an albumin of 2.0 corrects to $7.6 + 0.8 \ imes 2 = 9.2$ mg/dL, squarely normal.

The 0.8 comes from a small 1973 study by Payne and colleagues, and it is a population regression slope rather than a physical constant. Published alternatives range from 0.7 to 1.0 depending on the cohort, and the equation as a whole assumes a normal pH, normal binding affinity and an albumin that is the only relevant binding protein. Acidosis reduces calcium binding to albumin and raises the ionised fraction; a paraprotein can bind calcium independently; heparin and citrate in the sample tube interfere.

Several large validation studies have found that the correction performs poorly in exactly the populations it gets used in most, particularly dialysis patients and the critically ill, where it misclassifies a substantial fraction of results in both directions. Where the ionised calcium actually matters, the reliable answer is to measure it directly on a blood gas analyser rather than to correct the total. This calculation is a screening adjustment, useful for deciding whether a low total calcium is worth chasing, and not a substitute for the direct measurement.