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Vitamin C - Deficiency Test

Vitamin C chemical assay measures plasma levels of vitamin C (ascorbic acid), a water-soluble vitamin required for collagen synthesis and cartilage and bone maintenance. Vitamin C also promotes iron absorption, influences folic acid metabolism, and may be necessary for withstanding the stresses of injury and infection.

This vitamin is present in generous amounts in citrus fruits, berries, tomatoes, raw cabbage, green peppers, and green leafy vegetables. Severe vitamin C deficiency, or scurvy, causes capillary fragility, joint abnormalities, and multiple systemic symptoms.

Purpose

• To aid diagnosis of scurvy, scurvy-like conditions, and metabolic disorders, such as malnutrition and malabsorption syndromes.

Patient preparation

• Explain to the patient that this test detects the amount of vitamin C in the blood.
• Instruct him to observe an overnight fast before the test.
• Tell him that this test requires a blood sample. Explain who will perform the venipuncture and when.
• Reassure him that although he may feel some discomfort from the needle puncture and the pressure of the tourniquet, collecting the sample takes only a few minutes.

Procedure and posttest care

• Perform a venipuncture, and collect the sample in a 7-ml green-top (heparinized) tube.
• If a hematoma develops at the venipuncture site, apply warm soaks.
• Remove diet restrictions.

Precautions

• Avoid rough handling or excessive agitation of the sample to prevent hemolysis.
• Send the sample to the laboratory immediately.

Reference values

Normal plasma vitamin C levels range from 0.6 to 2 mg/dl.

Abnormal findings

Values less than 0.3 mg/dl indicate significant deficiency. Vitamin C levels diminish during pregnancy to a low point immediately postpartum. Depressed levels occur with infection, fever, and anemia. Severe deficiencies result in scurvy.

High plasma levels can indicate increased ingestion of vitamin C. Excess vitamin C is converted to oxalate, which is excreted in the urine. Excessive concentration of oxalate can produce urinary calculi.

Interfering factors

• Failure to observe pretest restrictions.
• Hemolysis due to rough handling of the sample.
• Failure to promptly send the sample to the laboratory.
• Tetracycline (possible decrease in leukocyte absorbate and increase in urinary loss of vitamin C)