What is ionised calcium?
Calcium in your blood comes in two forms. About half sits bound to protein, almost all albumin, and a small part to salts; that calcium is in transit and does nothing. The other half is free, ionised, and that is the form that counts: nerve cells fire with it, muscle fibres contract with it, your heart keeps its rhythm with it and your blood clots with it. An ordinary calcium test adds both forms together; this test measures only the free part. Your body guards exactly that free part. If it falls, your parathyroid glands release PTH within minutes, which takes calcium from your bones, makes your kidneys hold on to calcium and activates vitamin D so your gut absorbs more. If it rises, the PTH stops. Vitamin D is the raw material in that: without enough vitamin D your gut absorbs too little calcium, and then PTH has to keep taking it from the bones. Acidity joins in too: in more acidic blood albumin releases calcium and the free part rises, in more alkaline blood, as with fast breathing, albumin binds more and it falls. Your report shows ionised calcium in mmol/L. Normal is about 1.15 to 1.30 mmol/L, where the two labs that carry the test overlap. The band is narrow, because your body regulates free calcium so tightly.
Why is ionised calcium relevant?
Free calcium is the part that matters physiologically, and exactly that part is measured least. Almost every blood test measures total calcium, and that works well as long as your albumin is normal, because then the free part is a fixed fraction of the total. If your albumin deviates, with a liver disease, a kidney disease, malnutrition, a long-lasting inflammation or a pregnancy, the total no longer says anything about the free part: a low albumin gives a low total with a normal free calcium, and a high albumin from concentrating the reverse. The usual solution is a calculation, corrected calcium; the direct solution is measuring the free calcium. That makes the test useful in a few situations. With symptoms that point to a low calcium, tingling around the mouth and in the fingers, muscle cramps, a cramped hand, while the total is normal. With a high total calcium you want to know is real, before an investigation of the parathyroid glands begins. With a markedly deviating albumin. And with doubt about a calculation, because a formula based on albumin assumes an average body. For anyone who is healthy and has a normal albumin, the test adds little to total calcium, and that is the reason only two of the four labs carry it. Anyone who has it measured should know the draw matters: a tight tourniquet, a clenched fist and a tube left open for long shift the result.
Ionised calcium reference ranges
What counts beside the number:
- Albumin from the same draw, which says whether total calcium could be trusted
- PTH, which says whether the parathyroid glands are the driver of a high or low free calcium
- Vitamin D and magnesium, the most common reasons for a low free calcium
- Creatinine and the eGFR, because kidneys that work less lower free calcium
- How the draw went: a tight tourniquet, a clenched fist or fast breathing shift the result
- Whether you take calcium tablets or vitamin D, and in which dose
There is no national reference value for free calcium, even though it is the fraction that matters most physiologically and that your body keeps constant with PTH, active vitamin D and calcitonin. The band is narrow, and the draw matters: a tight tourniquet, a clenched fist, fast breathing and a tube left open shift the result.
What Dutch labs actually use
With ionised calcium the finding is not the spread but the availability. Of the four labs this site compares, only two carry the test, and those two sit close together: Unilabs 1.15 to 1.30 mmol/L, Certe 1.15 to 1.33. Star-shl and Clinical Diagnostics do not have the test in their catalogue, and the two that do are not measuring it the same way: at Unilabs in the central laboratory, at Certe beside the treatment room.
1.15 – 1.30mmol/L
Between 1.30 and 1.33 it depends on the lab, because each sets its limits on its own method and population. Above 1.33 every lab calls it abnormal.
every lab calls 1.15 – 1.30 mmol/L normal. Between 1.30 and 1.33 it depends on the lab.
Ionised calcium high or low: what it means
A low free calcium means too little is available for your nerves, muscles and clotting, and your regulating loop is not keeping up. Usually it is the first and rarely the last: a vitamin D deficiency, through which your gut absorbs too little calcium and PTH has to keep taking it from your bones; too little PTH, usually after an operation on the thyroid or the neck in which the parathyroid glands were damaged; and a reduced kidney function, in which the kidney no longer activates vitamin D and holds on to phosphate that binds calcium. A low magnesium also lowers free calcium, because the parathyroid glands release no PTH without magnesium. The symptoms are tingling, cramps and a heart that beats irregularly, and with a fast fall they come sooner than with a slow one. A high free calcium means there is more calcium in your blood than the regulating loop allows, and the question is whether PTH is the driver. A parathyroid gland releasing too much PTH is the most common reason in anyone who otherwise feels well, often found by chance, and you recognise it by a high free calcium beside a PTH that is not low. Calcium coming from somewhere else, from a tumour that breaks down bone or makes a substance resembling PTH, from too much vitamin D or from calcium tablets, gives a high free calcium with a low PTH. Rare is prolonged immobility, in which bones that are not loaded release calcium. What you do about it: with a low free calcium from a vitamin D deficiency you top up vitamin D, and the calcium follows in weeks; calcium from dairy, green vegetables, nuts and fish helps along. With a low magnesium you top that up first. With too little PTH a combination of calcium and active vitamin D is the treatment, and a doctor sets that. With a high free calcium from a parathyroid gland an operation is the solution once the calcium is high enough or the bones and kidneys suffer from it; until then drinking plenty, exercise and no calcium tablets help. With a high free calcium from vitamin D or calcium tablets, stopping is enough. Ionised calcium belongs beside albumin, because that says whether total calcium could be trusted, beside PTH, which says whether the parathyroid glands are the driver, and beside vitamin D, magnesium and creatinine, which show the most common reasons for a low free calcium.
What lowers it
Too little available calcium, and the regulating loop is not keeping up.
| Cause | How often |
|---|---|
| A vitamin D deficiencyThe gut absorbs too little calcium and PTH has to keep taking it from the bones; the most common reason, and restored with vitamin D in weeks | Often |
| Too little PTHUsually after an operation on the thyroid or the neck in which the parathyroid glands were damaged; a low magnesium also halts PTH release | Sometimes |
| Reduced kidney functionThe kidney no longer activates vitamin D and holds on to phosphate that binds calcium; creatinine and the eGFR show that | Sometimes |
What raises it
More calcium than the regulating loop allows, and PTH says whether the parathyroid glands are the driver.
| Cause | How often |
|---|---|
| A parathyroid gland releasing too much PTHThe most common reason in anyone who otherwise feels well, often found by chance; recognisable by a PTH that is not low beside a high calcium | Sometimes |
| Calcium coming from somewhere elseA tumour that breaks down bone or makes a substance resembling PTH, too much vitamin D or calcium tablets; the PTH is then low | Sometimes |
| Prolonged immobilityBones that are not loaded release calcium; moving and loading reverses it | Rare |
How does an ionised calcium blood test work?
- Referral
- Not needed, but not every lab carries the test: of the four large labs two measure it. Ask whether it is possible, and otherwise ask for total calcium with albumin.
- Fasting
- Not needed, though most labs prefer to measure fasting to keep the acidity and the albumin stable.
- When
- At a calm moment, without hard exertion beforehand. Skip calcium tablets and vitamin D on the day of the draw if you want to know your usual level.
- The draw
- One tube of blood from a vein in your arm, with a loose tourniquet and a relaxed hand, because congestion and pumping shift the free calcium. The tube stays closed and goes to the lab quickly, because air changes the acidity and with it the result.
- Where to draw
- You can draw at 250+ locations near you, no referral needed. See the locations
Frequently asked questions
What is ionised calcium?
The free part of the calcium in your blood. About half your calcium is bound to protein, mainly albumin, and is in transit; the other half floats free and is the form your nerves fire with, your muscles contract with, your heart keeps its rhythm with and your blood clots with. An ordinary calcium test adds both forms together; this one measures only the part that works.
What is a normal value for ionised calcium?
Between 1.15 and 1.30 mmol/L, where the two labs that carry the test overlap. The band is narrow, because your body adjusts free calcium within minutes with PTH and vitamin D. The draw matters: congestion, pumping the fist and fast breathing shift the result, so a calm draw gives your true value.
What is the difference from ordinary calcium?
Ordinary calcium is the total, bound and free together. As long as your albumin is normal, the free part is a fixed fraction of it and the total says the same. If your albumin deviates, through a liver disease, a kidney disease, malnutrition, a long-lasting inflammation or a pregnancy, the total distorts: a low albumin gives a low total with a normal free calcium. Measuring free calcium bypasses that problem instead of correcting for it.
What does a high ionised calcium mean?
That there is more calcium in your blood than the regulating loop allows. In anyone who otherwise feels well, a parathyroid gland releasing too much PTH is the most common reason, often found by chance and recognisable by a PTH that is not low. Calcium coming from somewhere else, from a tumour, too much vitamin D or calcium tablets, gives a high free calcium with a low PTH. A high calcium you take seriously, even without symptoms.
What does a low ionised calcium mean?
That too little calcium is available for your nerves, muscles and clotting. Usually it is a vitamin D deficiency, through which your gut absorbs too little calcium; sometimes too little PTH after a neck operation, kidneys that work less well or a low magnesium. You notice it as tingling around the mouth and in the fingers, muscle cramps and a heart that beats irregularly, and sooner with a fast fall than with a slow one.
Why is ionised calcium measured so rarely?
Because total calcium says the same with a normal albumin and is much simpler to measure. Free calcium needs a tube that stays closed and goes to the lab quickly, because air changes the acidity and with it the result. Two of the four large labs therefore do not carry the test. For anyone who is healthy with a normal albumin it adds little; it counts when albumin deviates or the total does not fit the symptoms.
When does ionised calcium add value over total calcium?
With a deviating albumin, because then the total no longer says anything about the free part. With symptoms that point to a low calcium while the total is normal. With a high total calcium you want to know is real, before an investigation of the parathyroid glands begins. And with doubt about corrected calcium, because that calculation assumes an average body and is often off in people who are ill.
How do I get my free calcium back up?
With a low free calcium from a vitamin D deficiency you top up vitamin D, and the calcium follows in weeks; dairy, green vegetables, nuts and fish help along, and a low magnesium you top up first. With too little PTH a doctor sets calcium with active vitamin D. With a high free calcium from a parathyroid gland an operation is the solution once needed, and until then drinking plenty, exercise and no calcium tablets help. With too much vitamin D or calcium tablets, stopping is enough.
Related biomarkers
Sources
- 1.Unilabs, test catalogue (Dutch): ionised calcium, serum. 2026. bepalingenklapper.nl
- 2.Certe, test catalogue (Dutch): ionised calcium (point of care). 2026. bepalingenwijzer.certe.nl
- 3.LESA Laboratory diagnostics (Dutch primary-care laboratory agreement). 2026. richtlijnen.nhg.org
- 4.NHG guideline on chronic kidney disease (M109), Dutch College of General Practitioners. 2026. richtlijnen.nhg.org
Educational information only, not medical advice. Consult a healthcare professional for clinical decisions.
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