What this calculator does
Most calcium in the blood travels bound to albumin, and only the unbound portion is biologically active. A total calcium measurement counts both, so when albumin is low the total reading understates how much active calcium is actually present.
The correction can change the interpretation entirely. A measured 8.50 mg/dL looks low, but against an albumin of 2.0 g/dL it corrects to 10.10 mg/dL, near the top of the normal range rather than below the bottom of it.
The formula
For every 1 g/dL that albumin falls below 4.0, add 0.8 mg/dL to the measured calcium. This is the Payne formula, the most widely used of several similar adjustments.
| Term | Meaning |
|---|---|
| Total calcium | What the laboratory measures: bound and unbound calcium together. |
| Ionised calcium | The unbound, active fraction, which can be measured directly but is not on a routine panel. |
| Payne formula | The standard albumin correction, adding 0.8 mg/dL per 1 g/dL of albumin deficit. |
The inputs explained
| Field | What to enter |
|---|---|
| Measured total calcium (mg/dL) | Measured total calcium in mg/dL from a standard panel. |
| Serum albumin (g/dL) | Serum albumin in g/dL. The formula assumes a normal value of 4.0. |
When to use it
Reading a panel with low albumin
Chronic illness, liver disease and malnutrition all lower albumin and distort the calcium reading.
Following a trend over time
Correcting each reading makes a series comparable when albumin has also been moving.
Understanding why two results disagree
A total calcium and an ionised calcium can point in different directions, and albumin is usually why.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does albumin shift the result?
The same measured calcium against three albumin levels.
| Serum albumin | Corrected calcium | Measured calcium |
|---|---|---|
| 2.0 g/dL | 10.10 mg/dL | 8.50 mg/dL |
| 3.0 g/dL | 9.30 mg/dL | 8.50 mg/dL |
| 4.0 g/dL | 8.50 mg/dL | 8.50 mg/dL |
Questions
How reliable is the correction?
It is an approximation, and its accuracy has been questioned in several studies, particularly in critically ill patients and in kidney disease. Where the answer really matters, a direct ionised calcium measurement is preferred.
Why does albumin matter at all?
Because roughly 40 per cent of blood calcium is bound to albumin and is not biologically active. Less albumin means less bound calcium, so a normal active level shows up as a low total.
Is there an equivalent in SI units?
Yes. In mmol/L with albumin in g/L, the common form adds 0.02 mmol/L for every 1 g/L that albumin sits below 40. This calculator uses the mg/dL and g/dL convention.
What if the corrected value is abnormal?
It should be discussed with the clinician who ordered the test. Calcium abnormalities have many causes and the correction is a screening adjustment, not a diagnosis.
For the unmeasured ion balance on the same panel, see the anion gap calculator. For estimated kidney function, see the creatinine clearance calculator.