Calcium is a mineral that is necessary for many important body processes, including the building and repair of bones, muscle contraction, heart function, nerve transmission, and blood clotting. Measuring blood levels of calciumand other substances that affect calcium metabolism through their interactionscan help to evaluate bone diseases, as well as the function of the kidneys and parathyroid glands.
Calcium in the body is primarily stored in the bones; the remainder travels in the bloodstream. Normally, parathyroid hormone and vitamin D carefully control the amount of calcium (and phosphorus) in the body by regulating the absorption of calcium from food, the amount removed from the body by the kidneys, and the amount mobilized from the bone. Abnormal blood calcium levels may indicate a problem somewhere in this system.
Parathyroid hormone (PTH) is secreted by the parathyroid glands when blood calcium levels are low. PTH acts to release calcium reservoirs from the bone into the blood; when blood calcium returns to normal levels, PTH secretion declines. PTH is typically measured to distinguish parathyroid from nonparathyroid causes of abnormal blood calcium. For example, high blood calcium is most commonly caused by elevated PTH due to hyperparathyroidism (overactive parathyroid glands).
Vitamin D (in its active form, or 1,25-dihydroxyvitamin D) is necessary for the absorption of dietary calcium by the intestine. When blood calcium is low, PTH induces the production of vitamin D to help the intestines to absorb more calcium, and stimulates the bones to release calcium into the blood. Low vitamin D levels—which can be caused by dietary deficiency, impaired absorption, or inadequate sunlight exposure (since this vitamin is synthesized from the skin upon exposure to ultraviolet light)—can lead to low calcium levels and bone disease.
Phosphorus is a mineral important for the building of bone and energy production. PTH and vitamin D regulate phosphorus levels. So, if disorders such as hyperparathyroidism or vitamin D deficiency are suspected, a blood test for phosphorus levels may be done. In chronic kidney (renal) failure, phosphorus levels are elevated because of inadequate excretion by the kidney.
Purpose of the Calcium Tests
- To evaluate calcium metabolism and parathyroid function
- To aid in the diagnosis of neuromuscular, skeletal, and endocrine disorders, arrhythmias, blood clotting dysfunction, and acid-base imbalance in the blood
- To help diagnose and evaluate a number of conditions that affect the bones, heart, nerves, kidneys, and teeth
- To monitor people with chronic renal failure, hyperparathyroidism, and certain types of malignancies
- To aid in the evaluation of low or high blood calcium levels (hypocalcemia or hypercalcemia)
- To confirm a suspected diagnosis of hyperparathyroidism
- To monitor patients with chronic renal failure
- To monitor patients with abnormal calcium or phosphorus levels and suspected PTH abnormality
- To evaluate bone diseases including bone malformation in children (rickets) and bone weakness, softness, or fracture (osteomalacia) and bone loss (osteoporosis) in adults
- To aid in the evaluation of abnormal calcium levels
- To detect overdose of vitamin D or monitor therapy with vitamin D
- To aid in the interpretation of the other tests
Who Performs Calcium Tests
- A nurse or a technician
Special Concerns about Calcium Tests
- Half of the calcium in the blood is bound to proteins (primarily albumin); the other half is known as free or ionized calcium. In most cases, the total amount of calcium (bound plus free) is measured. However, in people with low albumin levels—for example, malnourished patients—free calcium must be measured directly.
- A variety of medications may affect the test results.
- Blood levels of PTH, which vary during the day, are highest at around 2 AM and lowest at 2 PM. This pattern differs in people who work nights and sleep during the day; this fact must be taken into account when interpreting the test results.
Before the Calcium Tests
- Do not eat or drink anything after midnight on the day before a PTH or phosphorus test.
- Do not eat or drink anything for 4 hours before a vitamin D test.
- No fasting is required before a blood calcium test. However, if this test is performed together with a PTH, vitamin D, or phosphorus study, you must follow the food and drink restrictions required for the other tests.
What You Experience
- A sample of your blood is drawn from a vein, usually from the inside of the elbow or the back of the hand, and sent to a laboratory for analysis.
Risks and Complications of Calcium Tests
After the Calcium Tests
- Immediately after blood is drawn, pressure is applied (with cotton or gauze) to the puncture site.
- You may resume your normal activities and any medications that were withheld before the test.
- Blood may collect and clot under the skin (hematoma) at the puncture site; this is harmless and will resolve on its own. For a large hematoma that causes swelling and discomfort, apply ice initially; after 24 hours, use warm, moist compresses to help dissolve the clotted blood.
Calcium Test Results
- The blood sample is sent to a laboratory for analysis. Your doctor will review the results and consider them along with your symptoms, your physical exam, and the results of other tests.
- Hypercalcemia (chronically elevated blood calcium) is most commonly caused by hyperparathyroidism due to a benign parathyroid tumor.
- Hypocalcemia (chronically low blood calcium) may be caused by malnutrition, intestinal malabsorption, kidney failure, and acute inflammation of the pancreas (pancreatitis).
- Abnormal results often necessitate additional studies in order to confirm a diagnosis.
The Johns Hopkins Consumer Guide to Medical Tests
Simeon Margolis, M.D., Ph.D., Medical Editor
Updated by Remedy Health Media