Total calcium is a laboratory parameter that reflects the total amount of calcium in blood serum. Calcium plays an important role in bone formation, the function of the musculoskeletal, nervous, and endocrine systems, as well as blood clotting.
The test may be recommended in the presence of symptoms or previously diagnosed kidney diseases, including kidney stones, diseases of the bones, thyroid and parathyroid glands, neurological disorders, and certain cancers, as well as to monitor treatment effectiveness. A calcium test may be included as part of an annual laboratory check-up.
Calcium is one of the body's main mineral elements. About 99% of the body's total calcium is found in the bones and teeth. A small amount is present in the blood and other tissues, where it is involved in muscle contraction, nerve signal transmission, heart function, and blood clotting.
Calcium levels are regulated primarily by parathyroid hormone (PTH), which is produced by the parathyroid glands, and by vitamin D, through its effects on the kidneys, bone tissue, and intestines. Through their interaction, the body maintains a stable calcium level in the blood.
In the blood, calcium is present in several forms: ionized, or free, calcium (the active form that directly participates in cellular function); calcium bound to blood proteins; and calcium in complexes with other substances.
A total calcium blood test measures the combined amount of all these forms. Laboratory test names may also use the term serum calcium, meaning calcium measured in blood serum.
A calcium test does not show how much calcium a person consumes in their diet and does not directly assess calcium stores in the bones. The result is used as part of a comprehensive laboratory evaluation and is interpreted together with symptoms and other test results. Depending on the purpose of the evaluation, a physician may additionally order parathyroid hormone (PTH), phosphorus, magnesium, albumin, 25-hydroxyvitamin D, and kidney function tests.
Calcium blood test: what does it show?
The test measures the total calcium level in the blood and helps determine whether the blood calcium level is elevated or decreased. The test is also used to monitor calcium levels during treatment. The diagnosis of the cause of an abnormal result is not based on this result alone, so it is interpreted together with symptoms and other tests.
Normal calcium levels in the blood
The term 'normal calcium level in the blood' refers to the reference range – the range against which the test result is compared. The limits of the range may vary depending on age, the testing method, and the equipment used by the laboratory.
Reference values are provided on the laboratory report, so the range stated on the report should be used as the primary reference. A result outside the reference range does not necessarily indicate disease. If necessary, a physician may recommend repeating the test, measuring ionized calcium, or performing additional testing.
High calcium in the blood: main causes
An elevated calcium level in the blood is called hypercalcemia. One of the most common causes of this condition is primary hyperparathyroidism – a disorder in which the parathyroid glands produce excessive amounts of parathyroid hormone (PTH). This leads to an increased calcium level in the blood and may affect the function of various organs and systems. The signs of primary hyperparathyroidism can vary and may resemble those of other diseases.
These may include:
- general symptoms: persistent fatigue, feeling unwell, frequent headaches, sleep disturbances, and decreased sex drive;
- changes in mood and thinking: difficulty concentrating, memory impairment, irritability, anxiety, depressed mood, or loss of interest in activities that were previously enjoyable;
- kidney-related symptoms: increased thirst, frequent urination, and kidney stones; prolonged disease may impair kidney function;
- changes in bone tissue: reduced bone strength, osteoporosis, and bone pain, including pain in the arms, legs, or forearms;
- digestive system symptoms: constipation, heartburn, or gastric reflux;
- cardiovascular symptoms: a sensation of heart palpitations, abnormal heart rhythm, and elevated or fluctuating blood pressure.
Primary hyperparathyroidism is not the only possible cause of hypercalcemia. Calcium levels may also increase due to:
- certain cancers and other diseases in which calcium enters the bloodstream more actively from the bones or is absorbed more extensively in the intestines;
- excessive use of calcium or vitamin D preparations;
- use of certain medications;
- prolonged, significant restriction of mobility;
- rare inherited disorders in which the body does not regulate calcium levels properly.
Causes of low calcium levels
A low calcium level in the blood is called hypocalcemia. Hypocalcemia may cause tingling or numbness around the mouth or in the fingers, muscle cramps, and spasms. A significant decrease in calcium levels may cause seizures or impaired heart function and requires urgent medical attention.
Possible causes include:
- insufficient production of parathyroid hormone (PTH), including after surgery in the neck area;
- low levels of vitamin D or magnesium;
- chronic kidney disease;
- impaired absorption of calcium and vitamin D in the intestines;
- acute inflammation of the pancreas, severe diseases, or use of certain medications.
Low total calcium may also be associated with a low albumin level. In this situation, the level of active ionized calcium is not necessarily decreased.
