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Introduction ⬇

Homocysteine (tHcy) is an amino acid resulting from the synthesis of cysteine from methionine and enzyme reaction of cobalamin and folate. Large quantities of homocysteine are excreted and assimilated in the blood plasma of patients with homocystinemia associated with:

  1. Increased risk for vascular disease.

  2. Increased risk for venous thrombosis.

  3. Elevated homocysteine with a direct toxic effect on endothelium.

  4. Elevated in folic acid deficiency and vitamin B12 deficiency. Folic acid deficiency is characterized by elevated plasma homocysteine; folic acid supplementation reduces plasma homocysteine. Elevated plasma homocysteine levels due to aberrant vitamin B12 respond favorably to vitamin B12 supplementation.

  5. Increased risk for pregnancy complications and neural tube defects.

This test measures the blood plasma level of homocysteine. It is useful for providing a functional assay for folic acid deficiency, and for diagnosing homocystinemia. Homocysteine is retained by persons with reduced kidney function.

Normal Findings ⬆ ⬇

4–17 μmol/L or 0.54–2.30 mg/L for fasting specimens

Procedure ⬆ ⬇

  1. Obtain a venous blood sample (red-topped tube). Serum or heparinized plasma is needed. Fasting is necessary (10–12 hours).

  2. Observe standard precautions. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered. Place the specimen in a biohazard bag.

  3. Place on ice immediately after drawing. Centrifuge immediately and freeze within 1 hour of collection.

Clinical Implications ⬆ ⬇

Increased or elevated homocysteine levels occur in the following conditions:

  1. Folic acid deficiency

  2. Abnormal vitamin B12 metabolism and deficiency

  3. Homocystinuria

Clinical Alert

Homocysteine values and their relation to CAD are controversial but are currently regarded as a weak risk factor for heart disease. The methionine load test is also currently investigative and has not yet been approved as a routine test

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose and blood-drawing procedure.

  2. The test requires fasting.

  3. Evaluate kidney function in patients with homocystinuria.

  4. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Allow the patient to eat and drink after blood is drawn.

  2. Review test results; report and record findings. Modify the nursing care plan as needed.

  3. Evaluate for other cardiovascular risk factors, compare test results, and monitor appropriately. Promote lifestyle changes accordingly.

  4. Monitor for folic acid or vitamin B12 deficiency and provide supplements as needed.

  5. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆

  1. Penicillamine reduces plasma levels of homocysteine

  2. Nitrous oxide, methotrexate deficiency, and azauridine increase plasma levels of homocysteine

  3. CKD

  4. Hypothyroidism