Calcium blood test: normal range and what the result means

Most of the body's calcium is in the bones and teeth; only a small part of it is in the blood, but that part is vitally important for the muscles, the nerves, the heart rhythm and clotting, which is why it is kept within a very narrow interval. About half of the calcium in the blood is bound to proteins, mostly albumin, and the other half is free, ionised and active. Because of that, total calcium depends on albumin: when albumin is low, total calcium looks decreased even though ionised calcium is normal - laboratories then calculate corrected calcium or measure ionised calcium directly. The calcium balance is regulated by parathyroid hormone (PTH), vitamin D and calcitonin: PTH and vitamin D raise calcium in the blood by taking it from the bones and from the gut. So a deviation in calcium most often means a problem with the parathyroid glands, with vitamin D, with the kidneys or with the bones, rather than simply diet. Calcium is tested together with albumin, phosphorus, PTH and vitamin D.

• When bone disease, osteoporosis or a parathyroid gland disorder is suspected
• In kidney or liver disease, and when absorption in the gut is poor
• With muscle cramps, numbness, weakness or heart rhythm disturbances
• When too much or too little vitamin D, or hyperthyroidism, is suspected
• In some cancers - calcium can be raised
• After a long period of lying down

Raised calcium (hypercalcaemia) is most often linked with overactive parathyroid glands, or with taking too much calcium and vitamin D. Other causes are a long period of lying down, hyperthyroidism, sarcoidosis and other infections that cause granulomas, Paget's disease, myeloma, bone metastases and some medicines. Severe hypercalcaemia affects the kidneys and the heart, so it is assessed urgently.

Causes of high calcium

• Overactive parathyroid glands (hyperparathyroidism)
• Too much calcium or vitamin D from supplements
• A long period of lying down, immobility
• Hyperthyroidism
• Sarcoidosis and infections that cause granulomas
• Paget's disease
• Myeloma, bone metastases
• Some medicines - thiazide diuretics, lithium

Symptoms that can occur with high calcium

• Thirst, frequent urination
• Nausea, constipation
• Tiredness, weakness, confusion
• Kidney stones, bone pain

What to discuss with a doctor

• Parathyroid hormone (PTH) - it tells a parathyroid gland cause apart from the others
• Vitamin D and phosphorus
• The supplements and medicines being taken
• Kidney function - creatinine
• If PTH is normal or low - further tests to find the cause, which the doctor decides on

Low calcium (hypocalcaemia)

Lowered calcium (hypocalcaemia) is most often linked with vitamin D deficiency, with low albumin (in which case the true calcium can be normal), with kidney failure, with underactive parathyroid glands, with magnesium deficiency, with liver disease, with poor absorption in the gut or with inflammation of the pancreas. The signs are numbness and cramps and, less often, rhythm disturbances.

• Vitamin D deficiency
• Low albumin - total calcium is low, ionised calcium can be normal
• Kidney failure
• Underactive parathyroid glands (hypoparathyroidism)
• Magnesium deficiency
• Liver disease
• Poor absorption in the gut
• Inflammation of the pancreas

Symptoms that can occur with low calcium

• Numbness around the mouth and in the fingers
• Muscle cramps, spasms
• Tiredness
• Heart rhythm disturbances - in severe cases

What to discuss with a doctor when calcium is low

• Albumin, and corrected or ionised calcium - whether the shortfall is real
• Vitamin D, PTH, magnesium, phosphorus
• Kidney function
• Conditions that interfere with absorption

Albumin, corrected and ionised calcium

About half of the calcium in the blood is bound to albumin, so when albumin is low the total figure looks decreased even though the active (ionised) part is normal. Corrected calcium is recalculated from the albumin value; more precisely - ionised calcium is measured directly. That is why, when albumin is low, the doctor assesses corrected or ionised calcium, and why albumin is one of the first things checked when total calcium comes back low.

Reading the result

Calcium is rarely read on its own: it is tested together with albumin, phosphorus, parathyroid hormone and vitamin D, because parathyroid hormone and vitamin D regulate it and albumin determines how much of the total is bound. Laboratory intervals differ, so a result is compared with the interval on your own report. A doctor usually repeats the test and goes over the supplements and medicines being taken; when parathyroid hormone is normal or low with a raised calcium, which further tests are needed to find the cause is for the doctor to decide. More on results in general: how to read blood test results.