Ionised calcium
Ionised calcium is the free part of the calcium in your blood: the part not bound to protein and therefore actually working in your nerves, muscles and clotting. It is the value your body itself guards, and the measurement that bypasses a deviating albumin instead of correcting for it.
Ionised calcium reference ranges
What does your value mean?
Normal: 1.15 to 1.30 mmol/L
Enter your value as it appears on your results. Range for adults.
Ionised calcium high or low: what it means
Ionised calcium too low
Too little available calcium, and the regulating loop is not keeping up.
| Cause | How often |
|---|---|
| The 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 |
| Usually after an operation on the thyroid or the neck in which the parathyroid glands were damaged; a low magnesium also halts PTH release | Sometimes |
| The kidney no longer activates vitamin D and holds on to phosphate that binds calcium; creatinine and the eGFR show that | Sometimes |
Ionised calcium too high
More calcium than the regulating loop allows, and PTH says whether the parathyroid glands are the driver.
| Cause | How often |
|---|---|
| The 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 |
| A tumour that breaks down bone or makes a substance resembling PTH, too much vitamin D or calcium tablets; the PTH is then low | Sometimes |
| Bones that are not loaded release calcium; moving and loading reverses it | Rare |
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.
How does an ionised calcium blood test work?
- Fasting. Every draw is fasted: nothing to eat or drink from midnight, only water. Appointments are therefore in the morning, until 12:00.It also keeps the acidity and the albumin stable, which this measurement is sensitive to.
- 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.
- 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.
- Repeating. With a deviating value after four to eight weeks, together with PTH and albumin. Anyone topping up vitamin D measures again after three months.
- Where to draw. You can draw at 250+ locations near you, no referral needed. See the locations
More about ionised calcium
Ionised calcium explained
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.
How do you read ionised calcium?
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. 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.
Frequently asked questions
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.
Related biomarkers
Sources 4
- 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
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