The formula
In mg/dL, with albumin in g/dL:
corrected calcium = measured calcium + 0.8 × (4.0 − albumin)
In SI units, with calcium in mmol/L and albumin in g/L:
corrected calcium = measured calcium + 0.02 × (40 − albumin)
Both say the same thing. Roughly forty per cent of plasma calcium travels bound to albumin, and the laboratory measures the whole lot. When the albumin falls, the bound fraction falls with it and the reported total drops — while the ionised fraction, the only part the body senses and the only part that causes symptoms, may not have moved at all.
A worked example
A patient on the surgical ward has a total calcium of 8.5 mg/dL and an albumin of 2.4 g/dL. The calcium is flagged low.
8.5 + 0.8 × (4.0 − 2.4) = 8.5 + 1.28 = 9.8 mg/dL — comfortably normal. Nothing needs treating. The albumin is the abnormality, not the calcium.
Four ways a calcium result misleads you
| Problem | What happens |
|---|---|
| Low albumin | The commonest. The total falls while the ionised does not. Correct it, as above. |
| Tourniquet and fist-clenching | Venous stasis concentrates protein and falsely raises the total calcium. Take the sample without prolonged compression. |
| Acid–base shifts | Alkalosis increases binding to albumin, so the ionised calcium falls while the total is unchanged. This is why the hyperventilating patient gets perioral tingling and carpopedal spasm with a “normal” calcium. |
| The wrong tube | EDTA contamination chelates calcium and gives a spuriously, sometimes impossibly, low result. Repeat before you treat. |
The limit of the correction
The corrected calcium is an estimate, and a rough one. It performs poorly in critical illness, in renal failure, in pregnancy and at the extremes of albumin. When the answer actually changes what you do — a crisis, a patient being listed for a parathyroidectomy, a tetanic patient after neck surgery — measure the ionised calcium rather than correcting the total.
Reference intervals used here
| Test | mg/dL | SI |
|---|---|---|
| Total calcium | 8.6 – 10.2 mg/dL | 2.15 – 2.55 mmol/L |
| Ionised calcium | 4.5 – 5.3 mg/dL | 1.12 – 1.32 mmol/L |
| Albumin | 3.8 – 4.4 g/dL | 38 – 44 g/L |
| Phosphate | 2.5 – 4.5 mg/dL | 0.81 – 1.45 mmol/L |
| PTH (intact) | 15 – 65 pg/mL | |
Reference intervals differ between laboratories. Always read a result against the interval printed on your own report.