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Vitamin D3 - Deficiency Test

Vitamin D3 (cholecalciferol), the major form of vitamin D, is endogenously produced in the skin by the sun's ultraviolet rays and occurs naturally in fish liver oils, egg yolks, liver, and butter.

This test, a competitive protein binding assay, determines serum levels of 25-hydroxycholecalciferol after chromatography has separated it from other vitamin D metabolites and contaminants. It's commonly combined with measurement of serum calcium and alkaline phosphatase levels.

Purpose

• To evaluate skeletal disease, such as rickets and osteomalacia
• To aid diagnosis of hypercalcemia
• To detect vitamin D toxicity
• To monitor therapy with vitamin D3

Patient preparation

• Explain to the patient that this test measures vitamin D in the body.
• Tell the patient that he needn't restrict food or fluids.
• Inform him that the test requires a blood sample. Tell him who will perform the venipuncture and when.
• Reassure him that although he may feel discomfort from the needle and the tourniquet, collecting the sample takes less than 3 minutes.
• Check for drugs that alter test results (corticosteroids or anticonvulsants). If they must be continued, note this on the laboratory slip.

Procedure and posttest care

• Perform a venipuncture, and collect the sample in a 7-ml royal-blue-top tube.
• If a hematoma develops at the venipuncture site, apply warm soaks.

Precautions

• Handle the sample carefully to prevent hemolysis.

Reference values

In summer, the range for serum 25hydroxycholecalciferol values is from 15 to 80 ng/ml; in winter, it's 14 to 42 ng/ml.

Abnormal findings

Low or undetectable levels may result from vitamin D deficiency, which can cause rickets or osteomalacia. Such deficiency may stem from poor diet, decreased exposure to the sun, or impaired absorption of vitamin D (secondary to hepatobiliary disease, pancreatitis, celiac disease, cystic fibrosis, or gastric or small-bowel resection). Low levels may also be related to various hepatic diseases that directly affect vitamin D metabolism.

Elevated levels (> 100 ng/ml) may indicate toxicity due to excessive self-medication or prolonged therapy. Elevated levels associated with hypercalcemia may be due to hypersensitivity to vitamin D, as in sarcoidosis.

Interfering factors

• Hemolysis due to rough handling of the sample.
• Anticonvulsants, isoniazid, mineral oil, corticosteroids, aluminum hydroxide, cholestyramine, and colestipol (possible decrease)