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Calcium

This test is used to measure serum levels of calcium, the most abundant mineral in the body. More than 98% of the body's calcium is found in bones and teeth, but relative concentrations in those structures may vary as the body maintains calcium balance. For example, when calcium concentrations in the blood fall below normal, calcium ions move out of the bones and teeth to restore the blood's calcium level. Because the body excretes calcium daily, regular ingestion of calcium in food (at least 1 g/day) is necessary for normal calcium balance

Purpose

• To aid diagnosis of neuromuscular, skeletal, and endocrine disorders; arrhythmias; blood-clotting deficiencies;and acid-base imbalance.

Patient preparation

• Explain to the patient that this test is used to determine blood calcium levels.
• Tell him that the test requires a blood sample. Explain who will perform the venipuncture and when.
• Explain that he may experience slight discomfort from the needle puncture and the tourniquet but that collecting the sample usually take less than 3 minutes.
• Inform him that he needn't restrict food or fluids before the test.

Procedure and posttest care

• Perform a venipuncture (without a tourniquet if possible), and collect the sample in a 7-ml red-top or red­ marble-top tube.
• If a hematoma develops at the venipuncture site, apply warm soaks.

Reference values

• Normally, serum calcium levels range from 8.9 to 10.1 mg/d1 in adults age 19 and older. Serum calcium levels are higher in children and can rise during phases of rapid bone growth

Abnormal findings

Abnormally high serum calcium levels (hypercalcemia) may occur in hyper­pllrothyroidism and parathyroid tumors, Paget's disease of the bone, multiple myeloma, metastatic carcinoma, multiple fractures, and prolonged immnbilization. Elevated levels may also result from inadequate excretion of "calcium, as in adrenal insufficiency and renal disease; from excessive calcium ingestion; and from overuse of overuse such as calcium carbonat.

Interfering factors

• Venous stasis due to prolonged tourniquet application (possible false­high).
• Excessive ingestion of vitamin D or its derivatives (dihydrotachysterol, calcitriol); use of androgens, calciferol­activated calcium salts, progestinsestrogens, and thiazide diuretics (possible increase)
• Acetazolamide, corticosteroids, mithramycin, chronic laxative use, excessive transfusions of citrated blood (possible increase or decrease).
• Increased or decreased serum protein levels . Active bone growth and formation in childhood (increase).