When a laboratory reports a low calcium in someone whose albumin is also low, the standard response is to correct one for the other. The calculation is on every clinical reference site, in every hospital handbook, and in every online calculator including ours:
corrected calcium (mg/dL) = measured calcium + 0.8 × (4.0 − albumin in g/dL)Almost all of them cite the same source: Payne and colleagues, British Medical Journal, 1973. That paper does not contain the coefficient 0.8.
What Payne actually published
The abstract gives the formula and its units in a single sentence:
"A simple formula for adjusting calcium concentration was derived from the regression equation of calcium on albumin. Adjusted calcium = calcium − albumin + 4·0, where calcium is in mg/100 ml and albumin in g/100 ml."
Subtract albumin once. That is a coefficient of 1.0, in the same units the modern formula uses, where the modern formula uses 0.8.
The trap that makes this easy to get wrong
A second, metric version is also in circulation, and it is the one most clinical pages quote:
adjusted calcium (mmol/L) = total calcium + 0.02 × (40 − albumin in g/L)It is tempting to assume the 0.8 and the 0.02 are the same number in different clothes, and that one of them must therefore be Payne's. Convert and see. One mg/dL of calcium is 0.2495 mmol/L, and one g/dL of albumin is ten g/L, so:
| mg/dL per g/dL | mmol/L per g/L | |
|---|---|---|
| The formula in use | 0.802 | 0.02 |
| Payne 1973 | 1.000 | 0.025 |
⚠️ So the two familiar forms — 0.8 and 0.02 — really are the same number as each other. Neither is Payne's. His coefficient converts to 0.025, not 0.02, and the figure in universal use is 80.2% of the one he published.
Where the difference lands
The gap between the two corrections is 0.2 × (4.0 − albumin). It is zero when albumin is normal and grows as albumin falls, so the two versions diverge most in hypoalbuminaemia — exactly where the correction matters.
| Albumin (g/dL) | Difference between the two corrections |
|---|---|
| 4.0 (normal) | 0.00 mg/dL |
| 3.0 | 0.20 mg/dL |
| 2.5 | 0.30 mg/dL |
| 2.0 | 0.40 mg/dL |
| 1.5 | 0.50 mg/dL |
Whether that changes a decision depends on where the patient sits relative to the reference interval. Against 8.5 to 10.2 mg/dL — the interval our own calculator uses — an albumin of 2.0 g/dL produces a flip at each end:
| Measured calcium | Formula in use (0.8) | Payne (1.0) |
|---|---|---|
| 6.60 mg/dL | 8.20 — low | 8.60 — normal |
| 8.30 mg/dL | 9.90 — normal | 10.30 — high |
⚠️ The reference interval is doing as much work here as the coefficient. Widen the top of the range to 10.5 and the second row stops flipping, while the first still does. Any worked example that omits the reference interval is incomplete; ours is named for that reason.
We could not find where 0.8 came from
The honest position is that we do not know. Payne's paper gives 1.0. The 0.8 in universal use is not derived in any source we could read, and the clinical pages that use it attribute it to Payne rather than to anything else. Two earlier papers from the same period are the obvious candidates and neither was reachable.
We are stating that as a gap rather than filling it. A coefficient used at every bedside, attributed to a paper that published a different one, with no traceable derivation, is a more interesting fact than any guess we could offer about its origin.
What this does not mean
It does not mean 0.8 is wrong and hospitals should switch. Payne derived his figure by regression on two hundred specimens from one laboratory in 1973, using the assay methods of that laboratory. A coefficient fitted that way is not a constant of nature; it is a property of a population and a method, and it should be expected to differ elsewhere. There is a substantial literature arguing that albumin correction performs poorly in general and that ionised calcium should be measured directly where the answer matters.
What it means is narrower and firmer: the citation is wrong. The formula in use may well be better than Payne's for a modern laboratory. It is not the formula in the paper it names.
What we changed on our own tool
Our corrected calcium calculator uses 0.8, and it cited Payne 1973 for it. The arithmetic stays — 0.8 is what clinical practice uses and the tool should agree with the laboratory reading the result. The citation does not: it now records that the coefficient in use is not the one that paper published, and that its origin is unestablished.
If you are using any corrected-calcium calculator, ours included, the practical advice is the one clinical chemists have been giving for years. Treat the corrected value as a screen rather than a result, and where the number would change management in a patient with low albumin, ask for an ionised calcium rather than adjusting a total one.
Where this comes from, and what will date it
The formula and its units were read from the authors' own abstract of Payne RB, Little AJ, Williams RB and Milner JR, "Interpretation of serum calcium in patients with abnormal serum proteins", British Medical Journal 1973;4(5893):643–646. The unit conversion uses calcium's molar mass of 40.078, and every figure above reproduces from those two inputs.
Two limits. We read the abstract, not the body: the full paper is available only as a page scan without a text layer, so the regression itself, the population and the assay method are described here from the abstract's own summary and not from the tables. And we could not establish the origin of 0.8; it remains an open question.
This is unlikely to date. A fifty-year-old paper will not change what it says, and a coefficient that has survived this long without a traceable derivation is not about to acquire one. What could change is practice: if direct ionised-calcium measurement becomes routine at the point of care, the whole correction becomes a historical curiosity rather than a daily calculation.
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