Calcium (Ca) in Blood

A test for calcium in the blood checks the calcium level in the body that is not stored in the bones. Calcium is the most common mineral in the body and one of the most important. The body needs it to build and fix bones and teeth, help nerves work, make muscles squeeze together, help blood clot, and help the heart to work. Almost all of the calcium in the body is stored in bone. The rest is found in the blood.

Normally the level of calcium in the blood is carefully controlled. When blood calcium levels get low (hypocalcemia), the bones release calcium to bring it back to a good blood level. When blood calcium levels get high (hypercalcemia), the extra calcium is stored in the bones or passed out of the body in urine and stool. The amount of calcium in the body depends on the amount of:

• Calcium you get in your food.
• Calcium and vitamin D your intestines absorb.
• Phosphate in the body.
• Certain hormones, including parathyroid hormone, calcitonin, and estrogen in the body.

Vitamin D and these hormones help control the amount of calcium in the body. They also control the amount of calcium you absorb from food and the amount passed from the body in urine. The blood levels of phosphate are closely linked to calcium levels and they work in opposite ways: As blood calcium levels get high, phosphate levels get low and the opposite is true.

It is important to get the right amount of calcium [at least 1000 mg (1 g) a day] in your food because the body loses calcium every day. Foods rich in calcium include dairy products (milk, cheese), eggs, fish, green vegetables, and fruit. Most people who have low or high levels of calcium do not have any symptoms. Calcium levels need to be very high or low to cause symptoms.

A blood calcium test may be done:

• To check for problems with the parathyroid glands or kidneys, certain types of cancers and bone problems, inflammation of the pancreas (pancreatitis), and kidney stones. Abnormal results on an electrocardiogram (EKG) test may be caused by high or low calcium levels.
• To see if your symptoms may be caused by a very low calcium level in the blood. Such symptoms may include muscle cramps and twitching, tingling in the fingers and around the mouth, muscle spasms, confusion, or depression.
• To see if your symptoms may be caused by a very high calcium level in the blood. Such symptoms may include weakness, lack of energy, not wanting to eat, nausea and vomiting, constipation, urinating a lot, belly pain, or bone pain.
• As part of a routine blood test.

A blood calcium test cannot be used to check for a lack of calcium in your diet or for the loss of calcium from the bones (osteoporosis). The body can have normal calcium levels even if your diet does not have enough calcium in it. Other tests, such as bone mineral density, check the amount of calcium in the bones.

Do not take calcium supplements for 8 to 12 hours before having a blood calcium test. Your doctor will tell you if you should not eat or drink anything before your test.

The health professional drawing blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure to the site and then a bandage.

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

There is very little chance of a problem from having blood sample taken from a vein.

• You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
• In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
• Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

A test for calcium in the blood checks the calcium level in the body that is not stored in the bones.

Normal

Normal values may vary from lab to lab.

Normal blood calcium values are higher in children because their bones are growing quickly.

An ionized calcium test checks the amount of calcium that is not attached to protein in the blood. The level of ionized calcium in the blood is not affected by the amount of protein in the blood.

High values

• High levels of calcium in the blood may be caused by being on bed rest for a long time, hyperparathyroidism, kidney disease, tuberculosis, or cancer that has spread to the bones. Certain cancers can make a substance that causes high blood calcium levels.
• High levels of calcium in the blood can be caused by eating a diet with too much vitamin D, vitamin A, or calcium. Eating large amounts of milk products or taking too many calcium medicines, such as antacids or vitamin supplements, can also cause high blood calcium levels.
• High levels of calcium in the blood can be caused by dehydration, sarcoidosis, chronic liver or kidney problems, Paget's disease, and Addison's disease.
• In rare cases, hyperthyroidism may cause high blood calcium levels.
• In a person with cancer, a high blood calcium level is an emergency. Treatment must be started immediately to prevent the person from becoming confused and dehydrated.

Low values

• Low levels of calcium in the blood can be caused by parathyroid gland (hypoparathyroidism) problems, problems with your intestines that stop your body from absorbing calcium and other nutrients from food (malabsorption syndrome), bone problems, kidney disease, acute pancreatitis, or low amounts of the protein albumin in the blood (hypoalbuminemia).
• Low ionized calcium levels may be caused by low magnesium levels.
• Pregnant women and older men may also have low calcium levels.

Reasons you may not be able to have the test or why the results may not be helpful include:

• Drinking too much milk or using too much antacid, calcium salt, or calcium supplement.
• Taking medicines, such as diuretics, acetazolamide (Diamox), albuterol, estrogen, birth control pills, corticosteroids, and some medicines that control seizures.
• Taking too much vitamin D, lithium, laxatives, theophylline, or aspirin.
• Having many blood transfusions in a short period of time.

• More than one blood test may be needed to see whether blood calcium levels are too high.
• Low blood levels of calcium may be caused by low levels of protein (albumin) in the blood, because about half of all calcium in the blood is attached to albumin. For this reason, an ionized calcium level (which is not attached to albumin) and a blood albumin level may also be measured. For more information, see the medical test Total Serum Protein.
• Other tests that may be done to find the cause of abnormal blood calcium levels include blood tests for parathyroid hormone (PTH), chloride, acid phosphatase, alkaline phosphatase, and vitamin D. For more information, see the medical tests Parathyroid Hormone and Alkaline Phosphatase.
• Since calcium affects how the heart works, a high or low blood calcium level may be the cause of abnormal results on an electrocardiogram (EKG) test. A blood calcium test will be done to help find the cause of the abnormal EKG results.
• Calcium levels can also be checked in the urine. For more information, see the medical test Calcium (Ca) in Urine.

Other Works Consulted

• Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.

• Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.

• Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.