Short answer: the usual calcium blood test measures total calcium, about half of which is bound to the protein albumin. When albumin is low, total calcium reads low even if the active, ionised calcium is fine, so doctors sometimes calculate a corrected calcium or measure ionised calcium directly. Calcium is controlled by parathyroid hormone (PTH) and vitamin D. mmol/L = mg/dL × 0.2495.
At a glance
| Measures | Total calcium in serum or plasma (ionised calcium is a separate test) |
|---|---|
| Units | mg/dL (US) or mmol/L (Europe); 1 mg/dL = 0.2495 mmol/L |
| LOINC | 17861-6 (total calcium), 1751-7 (albumin) |
| Corrected calcium | measured (mg/dL) + 0.8 × (4.0 − albumin in g/dL) |
| Controlled by | Parathyroid hormone, vitamin D, kidneys |
| Keep | Calcium with albumin from the same sample, and vitamin D if done |
Total, ionised and corrected calcium
About half of the calcium in blood is bound to proteins, mainly albumin; the rest is free (ionised) and is the part the body uses. A low albumin, common in illness or poor nutrition, lowers total calcium without lowering ionised calcium. A widely used estimate is corrected calcium (mg/dL) = measured calcium + 0.8 × (4.0 − albumin g/dL). Many laboratories now prefer measuring ionised calcium when it matters, because the formula is approximate.
How the result is written on the report
| Result on the report | LOINC code | Usual unit | Also seen as |
|---|---|---|---|
| Calcium, total, serum or plasma | 17861-6 | mg/dL | mmol/L (×0.2495) |
| Albumin, serum or plasma | 1751-7 | g/dL | g/L (×10) |
mmol/L = mg/dL × 0.2495: 9.5 mg/dL ≈ 2.37 mmol/L. Use the range on your report.
Keeping the results in your record
Keep calcium with albumin from the same sample, and with vitamin D and PTH if they were tested. Anpheros Daily charts each over time; the lab results tracker converts calcium between mg/dL and mmol/L.
What this page does not do
It explains the test and the correction formula. It does not interpret your calcium or recommend supplements; that depends on PTH, vitamin D, kidney function and your history.
Sources
Frequently asked questions
Why does albumin change my calcium result?
About half of blood calcium is carried by albumin. Low albumin lowers total calcium even when the active calcium is unchanged.
How do I convert calcium from mg/dL to mmol/L?
Multiply by 0.2495. For example, 10.0 mg/dL is about 2.50 mmol/L.
What is the difference between total and ionised calcium?
Total includes calcium bound to proteins; ionised is the free, active part. Ionised calcium is measured separately when precision matters.
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