Why does albumin matter for calcium?

About 40% of total calcium is bound to albumin and is not physiologically active. When albumin is low, total calcium reads low even if the active ionised fraction is normal. The Payne correction adjusts total calcium to a normal-albumin equivalent.

What is the Payne formula used here?

Corrected calcium in mmol/L equals total calcium plus 0.02 times (40 minus albumin in g/L), where 40 g/L is the reference albumin. In mg/dL the equivalent is total plus 0.8 times (4.0 minus albumin in g/dL).

How does pH change ionised calcium?

Calcium and hydrogen ions compete for albumin binding sites. Acidosis frees calcium and raises ionised levels, while alkalosis lowers them. The tool adjusts by roughly 0.05 mmol/L for every 0.1 pH unit away from 7.40, in the opposite direction to pH.

Is this as good as a measured ionised calcium?

No. A directly measured ionised calcium on a fresh anaerobic blood-gas sample is the gold standard and should be used whenever available. This estimate is for situations where that measurement is not at hand.

What is the normal ionised calcium range?

Most laboratories report a reference range of about 1.10 to 1.30 mmol/L for ionised calcium. Values below this suggest hypocalcaemia and above it hypercalcaemia, but always interpret with the clinical picture.

Ionised Calcium Estimator from Total Calcium and pH

When a blood-gas analyser is not available, ionised calcium often has to be estimated from a routine total calcium, the serum albumin, and the patient’s pH. This tool chains the three standard adjustments so you get a defensible estimate rather than a raw total that may be misleading.

How it works

Three steps are combined:

corrected_Ca = total_Ca + 0.02 × (40 − albumin_g_L)     (Payne)
iCa_raw      = 0.5 × corrected_Ca                        (≈50% ionised)
iCa_pH       = iCa_raw − 0.05 × ((pH − 7.40) / 0.1)      (pH correction)

The Payne step removes the effect of low albumin on the total. Roughly half of the albumin-corrected total is ionised at pH 7.40. The pH step then nudges the ionised value up in acidosis and down in alkalosis, because calcium competes with hydrogen ions for albumin binding.

Example and notes

A total calcium of 2.40 mmol/L with albumin 28 g/L and pH 7.40 corrects to about 2.64 mmol/L, giving an estimated ionised calcium near 1.32 mmol/L — higher than the raw total would suggest, because the low albumin had masked it. Treat the result as a guide only: a directly measured ionised calcium on a fresh, anaerobic sample is always preferred, and all values should be read alongside the clinical picture and trend.