Vitamins, minerals and amino acids
Calcium
The calcium in your blood; an abnormality is usually related to the parathyroid gland, vitamin D, the kidneys or the protein albumin.
Everything about this testWhat it measures
Calcium is mainly found in the bones and teeth; the small part in the blood is needed for muscles, nerves and blood clotting. This analysis measures total calcium in the blood: the free, active part plus the part bound to proteins, mainly albumin. Parathyroid hormone (PTH) and vitamin D keep the value within narrow limits, by releasing calcium from the bones or absorbing more calcium from the intestine. The result therefore says nothing about how much calcium is in your bones or how much you take in through food.
- Calcium that is too high (hypercalcemia), usually due to an overactive parathyroid gland (primary hyperparathyroidism) or too much vitamin D or calcium from tablets
- Calcium that is too low (hypocalcemia), for example with a deficiency of vitamin D or magnesium, reduced kidney function or an underactive parathyroid gland
- An apparently abnormal calcium due to an albumin that is too low or too high
High
A raised calcium (hypercalcemia) in someone without symptoms is most often due to an overactive parathyroid gland (primary hyperparathyroidism). Usually there is then a benign small growth in one of the parathyroid glands, which as a result makes too much parathyroid hormone (PTH). Other causes are high doses of vitamin D, a lot of calcium tablets or antacid tablets containing calcium, thiazide-type diuretics, lithium, an overactive thyroid, sarcoidosis and being bedridden for a long time. A slightly raised value often causes no symptoms; at higher values, thirst, frequent urination, tiredness, nausea, constipation, kidney stones and confusion can develop. Rarely: a malignant condition, such as metastases in the bone or multiple myeloma (Kahler's disease, a bone marrow disease).
Low
A low total calcium is usually due to a low albumin in the blood. The free, active calcium is then normal and calcium balance is not disturbed; that is why calcium is always assessed together with albumin. A truly low calcium fits a deficiency of vitamin D, reduced kidney function, a magnesium deficiency, poor absorption in the intestine or an underactive parathyroid gland, for example after surgery on the neck. Symptoms that may go with it are tingling in the fingers and around the mouth, muscle cramps and muscle twitches.
Preparation
- Do you take calcium tablets? On the day of the blood test, take them after the sample is taken.
If your result is abnormal
With a raised calcium, the GP repeats the measurement together with albumin, phosphate, kidney function and parathyroid hormone (PTH). If the result fits an overactive parathyroid gland, referral to an internist (specialist in internal medicine) usually follows. With a low calcium, the GP looks at albumin, vitamin D, magnesium and kidney function.
Reliability
The measurement of total calcium is accurate, but a single value is hard to interpret without albumin, because a large part of the calcium is bound to this protein. According to the Dutch Society for Clinical Chemistry (NVKC) and the Acute Boekje of the Dutch internists, an abnormal value is assessed together with albumin, phosphate, magnesium, kidney function and, if needed, PTH and vitamin D.
Limitations
- A low or high albumin changes total calcium, while the active calcium can be normal.
- A tourniquet left on too long during the blood draw and dehydration can raise the value slightly.
- Vitamin D supplements, calcium tablets, antacid tablets containing calcium, thiazide diuretics and lithium affect the value.
- Reference values differ between laboratories.
How common is this
An overactive parathyroid gland (primary hyperparathyroidism), the main cause of a raised calcium in people without symptoms, occurs in about 1 in 100 people. The condition often causes few symptoms, so a raised calcium is sometimes the first clue. A truly low calcium is rare in healthy adults; it is then usually a low albumin.
Who is this useful for
The result is most meaningful with symptoms that may fit calcium that is too high (thirst, frequent urination, tiredness, constipation, kidney stones) or too low (tingling, muscle cramps), with bone loss, with use of high doses of vitamin D or calcium, with use of thiazide diuretics or lithium and with reduced kidney function. In healthy people without symptoms, a slightly abnormal value is often due to the albumin or the blood draw. If you eat a vegan diet, for example, and want to know whether your calcium intake is sufficient, this measurement is of little use: the body keeps calcium in the blood stable, even with a low intake.
Downsides and risks
- A slightly abnormal calcium due to an abnormal albumin or the blood draw can lead to unnecessary worry or unnecessary further testing without a repeat measurement.
- A normal result can give false reassurance about calcium intake or bone strength; the blood value says nothing about these.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results