Calcium (Ca²⁺) Level

A calcium level is the concentration of total and ionized calcium in the blood to reflect parathyroid function, calcium metabolism, and malignancy activity.

The bulk of body calcium (99%) is stored in the skeleton and teeth, which act as huge reservoirs for maintaining blood levels of calcium. About 50% (0.50) of blood calcium is ionized; the rest is protein bound. Only ionized calcium can be used by the body in such vital processes as muscular contraction, cardiac function, transmission of nerve impulses, and blood clotting.

The amount of protein in the blood also affects calcium levels because 50% (0.50) of blood calcium is protein bound. Thus, a decrease in serum albumin will result in a decrease in total serum calcium. The decrease, however, does not alter the concentration of the ionized form of calcium. Measurements of ionized calcium are done during open heart surgeries, liver transplantations, and other operations in which large volumes of blood anticoagulated with citrate are given. These tests are also used to monitor kidney disease, kidney transplantation, hemodialysis, hyperparathyroidism, hypoparathyroidism, pancreatitis, and malignancy. Calcium levels are influenced by a variety of factors, including parathyroid hormone (PTH), calcitonin, vitamin D, and dietary calcium intake.

1. Obtain a 5-mL venous blood sample in a serum separator tube (SST) or red-topped tube; this will provide sufficient serum for this test.

2. Observe standard precautions. Be aware that heparinized samples are preferred for ionized calcium studies. Citrated ethylenediaminetetraacetic acid and oxalate give falsely low values and should not be used in the syringe.

3. Label specimen with the patient’s name, date, and test(s) ordered. Place specimens on ice, keep tightly capped, and deliver immediately to the laboratory.

1. Normal levels of total blood calcium, combined with other findings, indicate the following conditions:

   1. Normal calcium levels with overall normal results in other tests indicate no problems with calcium metabolism.

   2. Normal calcium and abnormal phosphorus values indicate impaired calcium absorption owing to alteration of PTH activity or secretion (e.g., in rickets, the calcium level may be normal or slightly lowered and the phosphorus level depressed).

   3. Normal calcium and elevated blood urea nitrogen (BUN) levels indicate the following:

      1. Possible secondary hyperparathyroidism: Initially, lowered serum calcium results from uremia and acidosis. The reduced calcium level stimulates the parathyroid to release PTH, which acts on bone to release more calcium.

      2. Possible primary hyperparathyroidism: Excessive amounts of PTH cause elevation in calcium levels, but secondary kidney disease causes retention of phosphate and concomitant lower calcium levels.

   4. Normal calcium with decreased serum albumin indicates hypercalcemia. Normally, a decrease in calcium is associated with a decrease in albumin.

2. Hypercalcemia (increased total calcium levels [>12 mg/dL or >3 mmol/L]) is caused by or associated with the following conditions:

   1. Hyperparathyroidism due to parathyroid adenoma, hyperplasia of parathyroid glands, or associated hypophosphatemia

   2. Cancer (PTH-producing tumors):

      1. Metastatic bone cancers; cancers of lung, breast, thyroid, kidney, liver, and pancreas

      2. Hodgkin lymphoma, leukemia, and non-Hodgkin lymphoma

      3. Multiple myeloma with extensive bone destruction, Burkitt lymphoma

      4. Primary squamous cell carcinoma of the lung, neck, and head

   3. Granulomatous disease (e.g., tuberculosis, sarcoidosis)

   4. Thyroid toxicosis

   5. Paget disease of bone (also accompanied by high levels of alkaline phosphatase)

   6. Idiopathic hypercalcemia of infancy

   7. Bone fractures combined with bed rest, prolonged immobilization

   8. Excessive intake of vitamin D, milk, antacids

   9. Kidney transplantation

   10. Milk-alkali syndrome (Burnett syndrome)

3. Hypocalcemia (decreased total calcium levels [<4.0 mg/dL or <1.0 mmol/L]) is commonly caused by or associated with the following conditions:

   1. Pseudohypocalcemia, which reflects reduced albumin levels. The reduced protein is responsible for the low calcium level because 50% of the calcium total is protein bound.

   2. Hypoparathyroidism due to surgical removal of parathyroid glands, irradiation, hypomagnesemia, gastrointestinal (GI) disorders, or renal wasting. The primary form is very rare.

   3. Hyperphosphatemia due to kidney disease, laxative intake, or cytotoxic drugs

   4. Malabsorption due to sprue, celiac disease, or pancreatic dysfunction (fatty acids combine with calcium and are precipitated and excreted in the feces)

   5. Acute pancreatitis

   6. Alkalosis (calcium ions become bound to protein)

   7. Osteomalacia (advanced)

   8. Kidney disease

   9. Vitamin D deficiency, rickets

   10. Malnutrition

   11. Alcoholism, hepatic cirrhosis

4. Increased ionized calcium levels occur in the following conditions:

   1. Hyperparathyroidism

   2. Ectopic PTH-producing tumors

   3. Increased vitamin D intake

   4. Malignancies

5. Decreased ionized calcium levels occur in the following conditions:

   1. Hyperventilation to control increased intracranial pressure (total Ca²⁺ may be normal)

   2. Administration of bicarbonate to control metabolic acidosis

   3. Acute pancreatitis (e.g., diabetic acidosis, sepsis)

   4. Hypoparathyroidism

   5. Vitamin D deficiency

   6. Magnesium deficiency

   7. Multiple organ failure

   8. Toxic shock syndrome

Pretest Patient Care

1. Explain purpose and procedure. Encourage relaxation.

2. Tourniquet application should be as brief as possible when drawing ionized calcium to prevent venous stasis and hemolysis.

3. Ensure that calcium supplements are not taken within 8–12 hours before the blood sample is drawn.

4. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. Thiazide diuretics may impair urinary calcium excretion and result in hypercalcemia (most common drug-induced factor).

2. For patients with renal insufficiency undergoing dialysis, a calcium ion–exchange resin is sometimes used for hyperkalemia. This resin may increase calcium levels.

3. Increased magnesium and phosphate uptake and excessive use of laxatives may lower blood calcium level because of increased intestinal calcium loss.

4. When decreased calcium levels are due to magnesium deficiency (as in poor bowel absorption), the administration of magnesium will correct the calcium deficiency.

5. If a patient is known to have or suspected of having a pH abnormality, a concurrent pH test with ionized calcium level should be requested.

6. Many drugs may cause increased or decreased levels of calcium. Calcium supplements taken shortly before specimen collection will cause falsely high values.

7. Elevated serum protein increases calcium; decreased protein decreases calcium.

Normal

• See Table 14.5.