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What is a Calcium Blood test?

A calcium blood test is one of the routinely performed metabolic tests to measure the amount of calcium circulating in blood. Calcium is the most abundant mineral found in body, with a major proportion (99%) stored in bones and teeth, and the remaining (1%) is found in bloodstream. A small percentage of calcium controls many important functions, such as contraction and dilation of blood vessels, muscle contraction and transmission of signals from one nerve to another.

Calcium balance in body is regulated by the parathyroid hormone (PTH)–vitamin D endocrine axis. PTH is secreted when low levels of calcium are detected in the blood, which further signals calcium from bones to be transferred to the blood in its ionised form. An increase in ionised calcium causes an increase in the production of calcitriol (the active form of Vitamin D) by kidneys, which results in increased absorption of calcium from the gut. A calcium blood test would thus help a clinician detect underlying problems related to bones, thyroid and parathyroid glands, kidney or gut.

  1. Why is Calcium Blood test performed?
  2. How do you prepare for Calcium Blood test?
  3. How is Calcium Blood test performed?
  4. What do Calcium Blood test results indicate?

A calcium blood test is performed to measure two types of calcium:

  • Total calcium: Calcium, that is both free and bound to circulating blood proteins, mainly albumin
  • Ionised or free calcium: Calcium, which is not bound to circulating blood proteins and is in biologically active form

A physician will recommend a calcium blood test if you exhibit symptoms of

Calcium blood tests are also recommended in case of symptoms of certain bone diseases; cancers of breast, lungs, kidneys, blood and neck; chronic kidney or liver disease; disorders of the parathyroid gland; excess thyroid hormone, either due to overactive thyroid gland or over supplementation of thyroid medicines; or abnormal vitamin D levels.

Special preparation is not required for solely a calcium blood test. However, the same blood sample may be used to test other parameters and may require you to fast, usually for eight hours. Inform your physician if you are taking any of the following drugs as they may cause an increase or decrease in your blood calcium levels and give misleading results: bicarbonates, bisphosphonates, lithium, vitamin A, cinacalcet, calcitonin, hydrochlorothiazide, furosemide, oestrogens or any steroids.

For this test, a blood sample is usually taken from a  vein of an elbow or wrist. The procedure lasts for less than 5 minutes.

A thin needle is inserted into the vein, which will cause a sharp pricking sensation for a few seconds, and a small amount of blood (usually 2-3 mL) is filled into a syringe or blood sample container.

Normal results: Here are the normal values for a calcium blood test:

  • Ionised calcium: 1.1-1.35 mmol/L (4.64-5.28 mg/dL)
  • Total calcium in adults: 8.5-10.3 mmol/L (8.9-10.1 mg/dL)

Abnormal results:

The following values of blood calcium may be obtained in case of Hypercalcaemia

  • Mild hypercalcaemia
    • Total calcium:10.5-11.9 mg/dL (2.5-3 mmol/L) 
    • Ionised calcium: 5.6-8 mg/dL (1.4-2 mmol/L)
  • Moderate hypercalcaemia
    • Total calcium: 12-13.9 mg/dL (3-3.5 mmol/L)
    • Ionised calcium: 8-10 mg/dL (2-2.5 mmol/L)
  • Hypercalcaemia crisis
    • Total calcium: 14-16 mg/dL (3.5-4 mmol/L)
    • Ionized calcium:10-12 mg/dL (2.5-3 mmol/L)

Symptoms of hypercalcaemia can be seen even at mildly increased levels, but some patients could be asymptomatic. In moderate hypercalcaemia, calcium stone formation in kidneys, skin, vessels, lungs, heart and stomach can occur. Severe hypercalcaemia can be a medical emergency wherein coma and cardiac arrest can occur.

Before undertaking a diagnostic workup, it is essential to be sure that true hypercalcaemia, not a false-positive laboratory test, is present. A false-positive diagnosis of hypercalcaemia is usually the result of a disruption of albumin synthesis. Hypercalcaemia is a long-term problem, and it is cost-effective to obtain several serum calcium measurements to adequately treat it.

Hypocalcaemia

If the results of a calcium blood test are lower than normal, it may indicate hypoparathyroidism, deficiency of vitamin D, deficiency of magnesium, pancreatitis or kidney disease. Although almost one-half patients in intensive care units are reported to have calcium concentrations of less than 2.1 mmol/L (8.5 mg/dL), most do not have a reduced ionised calcium level. Care must be taken while interpreting low calcium levels, as it can be reflective of a variety of severe or acute illnesses. In a patient with hypocalcaemia, it is also necessary to measure albumin levels as it helps to distinguish between true hypocalcaemia and false hypocalcaemia.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. This information is purely from an educational perspective and is in no way a substitute for medical advice from a qualified doctor.

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References

  1. Institute of Medicine (US) Committee to Review Dietary Reference Intakes for Vitamin D and Calcium; Editors: A Catharine Ross, Christine L Taylor, Ann L Yaktine, and Heather B Del Valle.
  2. Laxmaya Savva. Serum calcium measurement: Total versus free (ionized) Calcium. Indian Journal of Clinical Biochemistry. 2005;20(2):158-161.
  3. John T Potts Jr. Harrison’s Principles of Internal medicine. Diseases of the parathyroid gland and other hyper- and hypocalcemic disorders. 16th edition. 2252-2262
  4. Ruppe MD. Medications that affect calcium. Endocr Pract. 2011;17 Suppl 1:26-30. doi: 10.4158/EP10281.RA. PMID: 21134875