Type of test Blood; urine

Normal findings

∗ In elderly individuals, values tend to decrease.

Possible critical values

 Total calcium:  < 6 or > 13 mg/dL

Ionized calcium: < 2.2 or > 7 mg/dL

Test explanation and related physiology

 The serum calcium test is used to evaluate parathyroid function and calcium metabolism by directly measuring the total amount of calcium in the blood. Determination of serum calcium is used to monitor patients with renal failure, renal trans plantation, hyperparathyroidism, and various malignancies. It is also used to monitor calcium levels during and after large-volume blood transfusions.

About half the total calcium in the blood exists in its free (ionized) form, and about half exists in its protein-bound form (mostly with albumin). The serum calcium level is a measure of both. As a result, when the serum albumin level is low (as in malnourished patients), the serum calcium level will also be low and vice versa. As a rule of thumb, the total serum calcium level decreases by approximately 0.8 mg for every 1-g decrease in the serum albumin level. Serum albumin should be measured with serum calcium.

When the serum calcium level is elevated on at least three separate determinations, the patient is said to have hypercalcemia.

The most common cause of hypercalcemia is hyperparathyroid ism. Parathyroid hormone (PTH; p. 676) causes elevated calcium levels by increasing gastrointestinal (GI) absorption, decreasing urinary excretion, and increasing bone resorption. Malignancy, the second most common cause of hypercalcemia, can cause elevated calcium levels in two main ways. First, tumor metastasis (myeloma, lung, breast, renal cell) to the bone can destroy the bone, causing resorption and pushing calcium into the blood. Second, the cancer (lung, breast, renal cell) can produce a PTH like substance that drives the serum calcium up (ectopic PTH). Excess vitamin D ingestion can increase serum calcium by increasing renal and GI absorption. Granulomatous infections, such as sarcoidosis and tuberculosis, are associated with hypercalcemia.

Hypocalcemia occurs in patients with hypoalbuminemia. The most common causes of hypoalbuminemia are malnutrition (especially in alcoholics) and large-volume IV infusions. Large blood transfusions are associated with low serum calcium levels because the citrate additives used in banked blood for anticoagulation bind the free calcium in the recipient’s bloodstream. Intestinal malabsorption, renal failure, rhabdomyolysis, alkalosis, and acute pancreatitis (caused by saponification of fat) are also known to be associated with low serum calcium levels.

Urinary calcium can also be measured. Excretion of calcium in the urine is increased in all patients with hypercalcemia. Urinary calcium levels are decreased in patients with hypocalcemia. The test is helpful in determining the cause of recurrent nephrolithiasis.

Interfering factors

• Vitamin D intoxication may cause increased calcium levels.

• Excessive ingestion of milk may cause increased levels.

• Serum pH can affect calcium values. A decrease in pH causes increased calcium levels.

• Prolonged tourniquet time will lower pH and falsely increase calcium levels.

• There is normally a small diurnal variation in calcium, with peak levels occurring around 9 pm.

• Hypoalbuminemia is artifactually associated with decreased levels of total calcium.

* Drugs that may cause increased serum levels include alkaline antacids, androgens, calcium salts, ergocalciferol, hydralazine, lithium, PTH, thiazide diuretics, thyroid hormone, and vita min D.

* Drugs that may cause decreased serum levels include acetazolamide, albuterol, anticonvulsants, asparaginase, aspirin, calcitonin, cisplatin, corticosteroids, diuretics, estrogens, heparin, laxatives, loop diuretics, magnesium salts, and oral contraceptives.

 Procedure and patient care

 • See inside front cover for Routine Blood Testing.

• Fasting: verify with laboratory

• Blood tube commonly used: red

 Abnormal findings

 Increased levels (hypercalcemia)

- Hyperparathyroidism

- Nonparathyroid PTH producing tumor (e.g., lung or renal carcinoma)

- Metastatic tumor to the bone

- Paget disease of bone

- Prolonged immobilization

- Milk-alkali syndrome

- Vitamin D intoxication

- Lymphoma

- Granulomatous infections (e.g., sarcoidosis and tuberculosis)

- Addison disease

- Acromegaly

- Hyperthyroidism

Decreased levels (hypocalcemia)

- Hypoparathyroidism

- Renal failure

- Hyperphosphatemia secondary to renal failure

- Rickets

- Vitamin D deficiency

- Osteomalacia

- Malabsorption

- Pancreatitis

- Fat embolism

- Alkalosis

- Hypoalbuminemia

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