The form of vitamin D most easily and accurately measured is 25-hydroxy-cholecalciferol [vitamin D₃, 25(OH)D₃, cholecalciferol], a monohydroxylated form that leaves the liver for subsequent dihydroxylation by the kidney. Indirect measurement of vitamin D by serum alkaline phosphatase, calcium, and phosphorus determinations preceded 25(OH)D₃ assays and may still be used in the diagnosis of disorders of calcium metabolism.

Vitamin D aids in the maintenance of calcium-phosphorus balance and in the deposition of calcium and phosphorus in the bone. It also facilitates absorption of calcium and phosphorus from the small intestine and aids in the renal excretion of phosphorus.

• Excessive ingestion of vitamin D leads to elevated levels.
• Therapy with anticonvulsants and glucocorticoids may produce decreased levels.

• Differential diagnosis of hypercalcemia caused by parathyroid adenoma or vitamin D toxicity
• Confirmation of vitamin D deficiency as the cause of bone disease
• Confirmation of vitamin D deficiency caused by malabsorption syndromes, hepatobiliary disease, and chronic renal failure
• Evidence of interference with vitamin D levels as a result of anticonvulsant or steroid therapy

Nursing Care Before the Procedure

Client preparation is the same as that for any study involving the collection of a peripheral blood sample.

• It is recommended that anticonvulsant and steroid medications be withheld for 24 hours before the test, although this practice should be confirmed by the person ordering the study.

A venipuncture is performed and the sample collected in a red-topped tube. The sample should be handled gently to avoid hemolysis and transported promptly to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample.

• Resume medications withheld before the test.
• Decreased level: Note and report symptoms of deficiency or decreased level such as bone deformities. Instruct client to eat foods that are enriched with vitamin D. Administer ordered oral supplement.
• Increased level: Note and report symptoms of excesses or increased levels taken in medications or vitamin supplements, or both (intoxication, renal calculi, gastrointestinal intolerance). Discontinue the oral intake of vitamin D, and instruct client to avoid foods high in vitamin D.