Medically reviewed by Jordana Haber Hazan, MD
Calcium is an essential mineral that serves many roles in the body, including bone formation, muscle movement, nerve signaling, and heart rhythm regulation.
A calcium blood test may be ordered as part of a routine checkup or to screen for or monitor medical conditions affecting the bones, kidneys, or parathyroid gland (a set of tiny organs in the neck, right next to the thyroid gland).
This article defines low, high, and normal levels of calcium in the blood in healthy individuals. It also explores possible causes and symptoms of an abnormal calcium level and what to expect during a calcium blood test.
Calcium Blood Test Results and Possible Meanings
A calcium blood test measures the total calcium level in a person’s bloodstream. The total calcium level includes what are termed “bound” and “unbound” calcium.
Bound calcium is calcium attached to a protein, usually albumin (a protein made in the liver). Unbound calcium is not attached to a protein. It’s the active form of calcium, called free or ionized calcium.
In most cases, a total calcium blood level is ordered. In specific scenarios, however, an ionized calcium blood level may be requested—for example, in people who are malnourished, critically ill, or have chronic kidney disease.
Low Calcium Levels
A low calcium level (hypocalcemia) is less than 8.5 milligrams per deciliter (mg/dL).
Depending on the severity and duration of the hypocalcemia, symptoms range from mild to life-threatening. Frequent symptoms include numbness around the mouth, tingling in the hands and feet, and muscle cramps and spasms. More severe symptoms include seizures and heart failure.
Surgical hypoparathyroidism describes an underactive parathyroid gland and is the most common cause of low calcium levels. Surgical hypoparathyroidism may occur after an operation on the parathyroid gland, thyroid gland, or neck—for example, to treat head and neck cancer.
Function of the Parathyroid Gland
The parathyroid gland produces parathyroid hormone (PTH). Along with vitamin D, PTH interacts with a person’s bones, kidneys, and gut to maintain steady calcium levels in the bloodstream.
Other possible causes of hypocalcemia include:
- Vitamin D deficiency: Vitamin D influences calcium absorption in the gut. A vitamin D deficiency may be caused by poor sunlight exposure, malnutrition, malabsorption syndromes (e.g., celiac disease), or diseases that affect vitamin D metabolism (e.g., chronic kidney or liver disease).
- Magnesium deficiency: Low magnesium levels cause a decrease in PTH production, resulting in low calcium levels. Causes include chronic diarrhea, alcohol use, and certain drugs (e.g., the chemotherapy drug cisplatin).
- Pancreatitis: Inflammation and damage to the pancreas causes accumulation of calcium in the abdominal cavity, which lowers PTH levels, leading to a decline in blood calcium.
- Medications: Prolonged therapy with the antiseizure drug Dilantin (phenytoin) and certain drugs for osteoporosis (decreased bone mineral density and bone mass)—bisphosphonates, calcitonin, and Prolia (denosumab)—may cause hypocalcemia.
Normal Calcium Levels
A normal blood calcium level is between 8.5 and 10.5 mg/dL. Keep in mind that calcium reference ranges may differ slightly depending on the laboratory being used to process the blood test.
Also, although it’s unknown precisely why, blood calcium levels vary slightly among different age groups. Specifically, blood calcium levels are a bit higher in children than in adults.
Calcium Level by Age
A normal calcium level by age is:
- Birth to 3 months: 8.0 to 11.3 mg/dL
- 3 months to 6 months: 8.9 to 11.2 mg/dL
- 6 months to 1 year: 9.0 to 11.3 mg/dL
- 1 to 3 years: 8.9 to 11.1 mg/dL
- 4 to 11 years: 8.7 to 10.7 mg/dL
- 12 to 18 years: 8.5 to 10.7 mg/dL
- Over 19 years: 8.5 to 10.5 mg/dL
High Calcium Levels
A mildly high calcium level (hypercalcemia) is between 10.5 and 11.9 mg/dL. Moderate hypercalcemia is between 12.0 and 13.9 mg/dL, whereas severe hypercalcemia is greater than 14.0 mg/dL.
Most cases of high calcium levels are detected incidentally and cause no or mild symptoms like loss of appetite, nausea, constipation, fatigue, anxiety, and excessive urination (polyuria).
Long-term elevation of calcium levels or levels that continue to rise may cause weakness, weight loss, kidney stones, or kidney injury. Severe hypercalcemia is associated with life-threatening problems like heart arrhythmia (irregular or abnormal heartbeat), confusion, and coma.
There are several possible causes of high calcium blood levels, although the vast majority are due to either primary hyperparathyroidism or hypercalcemia of malignancy (cancer).
Primary hyperparathyroidism describes an overactive parathyroid gland, resulting in excess PTH production.
PTH causes an increase in blood calcium levels by the following actions:
- Bones releasing calcium into the bloodstream
- Intestines absorbing calcium from digested food
- Kidneys holding onto calcium, so it’s not removed in the urine
Hypercalcemia of malignancy affects an estimated 30% of people with cancer and is particularly common in individuals with advanced cancer.
The leading cause of hypercalcemia of malignancy is the release of parathyroid hormone-related protein (PTHrP) by cancer cells. This protein acts similarly to PTH, drawing calcium out into the bloodstream from the bone.
Another cause of hypercalcemia of malignancy is the spread (metastasis) of cancer cells into bone. This invasion into bone releases calcium into the bloodstream.
Cancers Linked to Hypercalcemia of Malignancy
In the United States, common cancers associated with hypercalcemia of malignancy are breast, lung, multiple myeloma, head and neck, kidney, ovarian, and certain lymphomas.
Other possible causes of hypercalcemia levels include:
- Medications (e.g., thiazide diuretics and lithium)
- Vitamin D or vitamin A toxicity
- Prolonged Immobilization/being bedridden
- Sarcoidosis (a disease in which clusters of inflamed tissue, called granulomas, invade organs)
- Hyperthyroidism (overactive thyroid gland)
- Addison’s disease (primary adrenal insufficiency)
- Rare genetic disorders like multiple endocrine neoplasia (MEN) syndrome type 1 or type 2A or familial hypocalciuric hypercalcemia (FHH)
Routine Blood Work With Calcium Biomarkers
Low and high calcium levels can be detected with a routine blood test called a basic metabolic panel (BMP) or a comprehensive metabolic panel (CMP).
What Does a BMP and CMP Measure?
A BMP measures the following:
- Calcium
- Blood glucose (sugar)
- Sodium
- Potassium
- Carbon dioxide
- Chloride
- Blood urea nitrogen (BUN)
- Creatinine
In addition to the components measured in a BMP, a CMP also measures:
- Albumin
- Total protein
- Bilirubin
- Liver enzymes
A BMP or CMP can be obtained at a healthcare provider’s office, clinic, hospital, or lab. It usually takes just minutes to draw the blood.
During a BMP or CMP blood draw, a phlebotomist performs the following steps:
- Identifies you and prepares labels for the blood tube(s)
- Locates and cleans the area of skin over a vein, usually inside the elbow
- Ties an elastic band around your upper arm to allow the vein to fill with blood
- Sticks a tiny needle (attached to a small test tube) into the vein to draw blood (may cause a mild stinging sensation)
- Removes the elastic band while the blood is being collected
- Removes the needle from the vein when the tube is filled with blood
- Applies gentle pressure with a gauze pad or cotton over the area of skin where the needle entered the vein
- Covers the gauze pad or cotton with a bandage, which can usually be removed within a few hours
- Ensures the tube is labeled accurately and sends it to the laboratory for testing
Related: Overview of Routine Blood Tests
Before Having Blood Drawn
There may be no preparation needed before undergoing a BMP or CMP. However, if your healthcare provider is evaluating lab values such as blood glucose, you may be asked to fast, which means not eating or drinking (except water) for 8 to 12 hours before the test.
Your healthcare provider may also ask you to stop taking certain medications, vitamins, or supplements to ensure the most accurate test results.
Next Steps With Calcium Blood Test Results
Although laboratories vary, BMP and CMP test results should return within 24 hours. Be sure to reach out to your healthcare provider to discuss your results.
If you have a low or high calcium level, your healthcare provider will likely repeat the test to confirm the abnormal finding. They may also order an ionized calcium level (the active form of calcium) to obtain a complete picture of your calcium status.
When a low or high calcium level is confirmed, a healthcare provider will order additional blood tests based on factors like symptoms, family history, and use of various medications, vitamins, and supplements.
The degree of calcium abnormality and how quickly the level rose or dropped (if prior calcium blood values are available) will also be considered.
Examples of such follow-up blood tests include:
- Parathyroid hormone level
- Magnesium level
- Phosphorus level (a mineral regulated by PTH, like calcium and magnesium)
- Vitamin D level
- PTH-related protein level
- Serum protein electrophoresis (a test that can help check for multiple myeloma)
Summary
Calcium is vital to bone health and muscle, nerve, and heart functioning. Calcium levels in the bloodstream can be measured with a routine blood test, usually a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP).
A low calcium level (hypocalcemia) is most commonly due to surgical hypoparathyroidism—surgery within the neck causing the parathyroid gland to become underactive. Additional causes include vitamin D and magnesium deficiency, among others.
The two most common causes of a high calcium level (hypercalcemia) are primary hyperparathyroidism—an overactive parathyroid gland—and hypercalcemia of malignancy, which is when cancer cells cause calcium from bones to leak into the bloodstream.
Speak with a healthcare provider if you have an abnormal calcium blood level. More blood tests are usually performed to confirm the finding and help diagnose the cause.
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