Blood calcium: what it is, what it tells you and why we test it

Blood calcium is the amount of calcium in your blood. Your body keeps it within very tight limits, because calcium keeps your muscles, nerves and heart working. The measured number also depends on albumin, the protein that carries part of it.

Why we test it

Almost all the calcium in your body sits in your bones. There is little in your blood, but it is needed all the time: it makes muscles contract, carries signals along nerves and keeps your heart beating.

That is why your body keeps it almost constant, with the parathyroids, four small glands in the neck, and with vitamin D. If the number moves outside the limits, there is usually a reason to find.

A slightly high calcium often causes no symptoms and is found by chance, on a blood test. Finding it early helps protect your bones and kidneys.

What it tells you, and what it does not

If it is high, overactive parathyroids are often behind it. Sometimes calcium or vitamin D supplements, or being dehydrated at the draw.

If it is low, albumin comes first: when albumin is low, the measured calcium looks lower than it really is.

And it does not show how much calcium is in your bones, or whether you eat enough of it: your blood keeps it steady either way.

That is why it is read together with other tests:

Normal calcium levels

The lab decides what is normal. It is usually 8.6 to 10.2 mg/dL, the same for women and men. Your own report may print different limits.

The limits are tight, because your body keeps calcium almost constant. A number just above the limit should still be shown to a doctor, while one just below often comes from albumin.

In the Rosa report you also see your optimal range, with albumin and vitamin D in the same panel.

What your result means

Find the row for your result, in mg/dL. Which values are dangerous? Above 10.5 or below 8 should always be seen by a doctor, above 12 or below 7.6 the same day, above 14 at the emergency department.

If your calcium is high or low

What causes high calcium

A high calcium, called hypercalcaemia, always needs explaining with a doctor. The main causes:

Symptoms of high and low calcium

A calcium just outside the limits usually causes no symptoms. It is often found by chance, on a blood test.

When it is high, thirst, passing urine often, constipation, nausea and tiredness can appear, and over the years kidney stones and weaker bones. With confusion, severe drowsiness, an irregular heartbeat or fainting, or above 12 with vomiting or severe thirst, call 112 or go to the emergency department.

When it is truly low, it can cause tingling around the mouth or in the fingers, and cramps or spasms in the hands and feet. With these, or with trouble breathing or a seizure, call 112 or go to the emergency department.

Low calcium (hypocalcaemia): what it means

Often it is not truly low. About half the calcium in your blood is free, and that is the part that works. Much of the rest is bound to proteins, mostly albumin. If albumin is low, the measured calcium drops, but the free calcium can be normal.

There are formulas that correct calcium for albumin, but in healthy people they can get it wrong. That is why checking it is your doctor’s call, and sometimes they ask for ionised calcium, the free part.

When calcium is truly low, vitamin D or magnesium may be very short, or the kidneys may filter less. Sometimes the parathyroids work too little, for example after thyroid surgery. It does not mean you should take calcium: first the reason has to be found.

What to do if your calcium is out of range

It depends on the direction and the cause. Where to start:

What can skew the result

A few things change the number for a while. It helps to know them before you read it.

When to worry

Talk to your doctor if:

With Rosa, a calcium above 10.5 mg/dL or below 8 is always flagged to a doctor, above 12 or below 7.6 urgently.

Frequently asked questions

What is the difference between total and ionised calcium? Total calcium, the number on your report, adds up free calcium and calcium bound to proteins. Ionised calcium is only the free part, the part that works: your doctor asks for it when the total is not enough.

Does calcium from food raise blood calcium? Usually not: your body keeps blood calcium steady whether you eat a lot of it or a little. A supplement taken the morning of the blood test, though, can raise it for a few hours.

If my calcium is low, should I take calcium? No, not on your own: it is first read with albumin. A truly low calcium is usually not about diet: vitamin D or magnesium may be very short, or the parathyroids or kidneys may be involved. Your doctor decides.

Does blood calcium show whether I have osteoporosis? No. Your bones can lose calcium while the blood number stays normal, because your body keeps it steady. Bones are checked with a different test, a bone density scan (DEXA).

Do I need to fast for a calcium test? It is better, also because other tests in the same draw usually need it. You can drink water. If you take calcium or vitamin D, tell your doctor.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it once a year. If it is out of range, your doctor decides when to repeat it.

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