Ionized calcium optimal range and reference range
Compiled by Alina Radu, founder of Sera, from the published sources below · Last reviewed November 9, 2026
Sera's bands for Ionized calcium:
- Optimal 1.15 – 1.35 mmol/L (4.6 – 5.4 mg/dL); out of range below 1.15 or above 1.35 mmol/L.
Evidence grade: Published reference interval. A band that primarily reflects a published adult laboratory or population reference interval. It is not necessarily applicable to every assay or laboratory.
All units
| Unit | Optimal | Sufficient | Out of range |
|---|---|---|---|
| mmol/L | 1.15 – 1.35 | — | below 1.15 or above 1.35 |
| mg/dL | 4.6 – 5.4 | — | below 4.6 or above 5.4 |
No separate Sufficient band is currently defined for this marker.
What the evidence says
Ionised calcium is the free, active fraction and is not affected by albumin. The band follows published laboratory intervals: 1.15 to 1.35 mmol/L in a UK NHS laboratory and 4.57 to 5.43 mg/dL at Mayo Clinic Laboratories. Other laboratories run higher, up to 1.40 mmol/L (5.6 mg/dL), and the result depends on the analyser and on whether it is adjusted to pH 7.4, so the interval printed on the report takes precedence. It is a laboratory interval, not an optimal target.
Sources
- CAI - Calcium, Ionized, Serum — Mayo Clinic Laboratories Test Catalog, 2026 · Laboratory / reference-interval sourceIonised calcium reference values for ages 24 to 97 are 4.57 to 5.43 mg/dL, about 1.14 to 1.36 mmol/L. This matches Sera's lower edge of 4.6 mg/dL and supports an upper edge of 5.4 mg/dL rather than 5.2. Current catalogue entry, opened October 2026.
- Calcium (ionised) — Liverpool Clinical Laboratories, Royal Liverpool University Hospital (pathlabs.rlbuht.nhs.uk), 2026 · Laboratory / reference-interval sourceReference range 1.15 to 1.35 mmol/L. Matches Sera's lower edge of 1.15 exactly and supports an upper edge of 1.35 rather than 1.30. The page is undated and does not name the trust in its text.
- Calcium - ionized — MedlinePlus Medical Encyclopedia, US National Library of Medicine, 2025 · Government / public-health guidanceAdult ionised calcium is 4.8 to 5.6 mg/dL, or 1.20 to 1.40 mmol/L. This is a higher interval than Sera's on both sides and shows that 1.30 mmol/L is below the upper limits in mainstream use.
- Calcium, Ionized — Labcorp Test Directory, 2026 · Laboratory / reference-interval sourceReference range 4.5 to 5.6 mg/dL. Agrees with Sera's lower edge of 4.6 mg/dL to within 0.1 and puts the upper limit at 5.6, above Sera's 5.2. Current directory entry, opened October 2026.
What Calcium (Ionised) measures
The free calcium in your blood that is not attached to albumin. This is the form your body actually uses, and it is the most accurate way to measure calcium status.
Ionised calcium is the reference method, so it settles the question when total or adjusted calcium is borderline. It runs on a much smaller scale than total calcium, roughly 1.15 to 1.30 mmol/L against 2.2 to 2.6, so the two numbers are not comparable. Many Romanian and European panels report this as their standard calcium row.