Corrected calcium for albumin

Adjust a total serum calcium for a low albumin, so that the result reflects the physiologically active fraction.

What is this for?

About 40% of serum calcium is bound to albumin. When albumin is low the total calcium falls even though the free, ionised, physiologically active calcium is unchanged. The correction estimates what the total calcium would be at a normal albumin.

How to use it

Enter the total serum calcium, choosing mg/dL or mmol/L from the unit dropdown.
Enter the serum albumin in g/dL from the same draw.
If an ionised calcium is available, use that instead — the correction is only a surrogate.

Worked example

A hospitalised patient has a total calcium of 7.8 mg/dL and an albumin of 2.5 g/dL. Is the patient truly hypocalcaemic?

Answer: 7.8 + [(4.0 − 2.5) × 0.8] = 7.8 + 1.2 = 9.0 mg/dL, which is within the normal range. The apparent hypocalcaemia is entirely explained by the low albumin.

Clinical pearls & pitfalls

This correction has been repeatedly shown to perform poorly against measured ionised calcium, particularly in critical illness and renal failure. Where the answer will change management, measure the ionised calcium.
Do not correct when an ionised calcium is already available — the correction adds error rather than information.
Acid–base status changes calcium binding independently of albumin: alkalosis increases binding and lowers ionised calcium, which is the mechanism behind tetany with hyperventilation. The albumin correction does not capture this.
In chronic kidney disease and after large citrate loads (massive transfusion, citrate anticoagulation), the correction is especially unreliable.

Assumptions & limitations

An estimate of the total calcium at normal albumin, not a measurement of ionised calcium.
Poor agreement with measured ionised calcium in critical illness, renal failure, and after transfusion.
Does not account for pH, which independently alters protein binding.
The 0.8 coefficient derives from a 1973 study in a specific population and does not hold universally.

References

Payne RB, Little AJ, Williams RB, Milner JR. Interpretation of serum calcium in patients with abnormal serum proteins. Br Med J. 1973;4(5893):643-646.
Steen O, Clase C, Don-Wauchope A. Corrected calcium formula in routine clinical use does not accurately reflect ionized calcium in hospital patients. Can J Gen Intern Med. 2016;11(3):14-21.
Kidney Disease: Improving Global Outcomes (KDIGO) CKD-MBD Work Group. KDIGO 2017 clinical practice guideline update for CKD-MBD. Kidney Int Suppl. 2017;7(1):1-59.