Why is calcium corrected for albumin?

About half of serum calcium is bound to albumin and is not biologically active. When albumin is low, the total calcium falls even though the active ionised calcium may be normal. Correcting for albumin estimates what the total calcium would be at a normal albumin level.

What is the corrected calcium formula?

In conventional units: corrected calcium = total calcium + 0.8 × (4 − albumin in g/dL). In SI units: corrected calcium = total calcium + 0.02 × (40 − albumin in g/L). The factor adds calcium when albumin is below its reference and subtracts it when albumin is above.

When is corrected calcium most useful?

It is most useful when albumin is abnormal, such as in critical illness, nephrotic syndrome, malnutrition, liver disease, or malignancy. It helps avoid both missing true hypercalcaemia or hypocalcaemia and over-treating an apparent abnormality that is really just low albumin.

Is corrected calcium as accurate as ionised calcium?

No. The gold standard for biologically active calcium is a direct ionised calcium measurement. The albumin correction is an estimate that becomes less reliable in acid-base disturbances, severe illness, and very abnormal albumin, so ionised calcium is preferred when available.

What reference albumin does the formula use?

The standard reference albumin is 4 g/dL (40 g/L). The correction adjusts the measured calcium toward what it would be if albumin were at this reference, so when albumin equals 4 g/dL the corrected and measured values are identical.

Corrected Calcium for Albumin

What corrected calcium means

Roughly half of the calcium in serum is bound to albumin and is biologically inactive; only the free, ionised fraction is physiologically important. When albumin is abnormal, the total (measured) calcium can be misleading — a low albumin lowers total calcium even when the active ionised calcium is normal. The albumin correction estimates what the total calcium would be if albumin were normal, giving a clearer picture of true calcium status.

How it works

The standard correction adjusts measured calcium toward a reference albumin of 4 g/dL (40 g/L):

Conventional (mg/dL, g/dL):  Corrected Ca = Total Ca + 0.8 × (4 − albumin)
SI units (mmol/L, g/L):      Corrected Ca = Total Ca + 0.02 × (40 − albumin)

For every 1 g/dL (10 g/L) that albumin falls below the reference, 0.8 mg/dL (0.2 mg/dL per g/dL × … or 0.02 mmol/L per g/L) of calcium is added back. When albumin is above the reference the formula subtracts calcium. If albumin equals the reference, corrected and measured calcium are identical.

Tips and example

A patient with a measured calcium of 8.4 mg/dL and albumin of 2.5 g/dL has corrected calcium = 8.4 + 0.8 × (4 − 2.5) = 8.4 + 1.2 = 9.6 mg/dL — a normal true calcium that the raw value made look low. The correction is only an estimate: in acid-base disturbances, severe illness, or markedly abnormal albumin, a direct ionised calcium measurement is more reliable and should be used where available.