section name header

Introduction ⬇

See foregoing cortisol test for purpose and indications. The DST helps to differentiate causes of elevated cortisol. Cortisol level less than 15 μg/dL (<41.4 nmol/L) is an indication of adrenal cortisol insufficiency.

Normal Findings ⬆ ⬇

<5 μg/dL or <138 nmol/L or <50% or 0.50 of baseline

Procedure ⬆ ⬇

  1. Obtain a 5-mL venous blood sample the day after administration of dexamethasone. Serum or heparinized plasma is acceptable. 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.

  2. Administer dexamethasone tablets in the late evening or at bedtime. There is a low-dose and high-dose suppression test in which either 1.0 mg or 8.0 mg of dexamethasone is given, respectively, at 11:00 p.m. (2300 hours). The following morning at 8:00 a.m. (0800 hours), a blood sample is drawn to measure cortisol. (Some patients with Cushing disease have false-positive results with the low dose.)

Clinical Implications ⬆ ⬇

  1. Suppression occurs in persons with:

    1. Cushing syndrome (>10 μg/dL or >276 nmol/L)

    2. Endogenous depression (50% of cases)

  2. No suppression occurs in:

    1. Adrenal adenoma, carcinoma

    2. Ectopic ACTH-producing tumors

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose and procedure. Fasting is required for the 8:00 a.m. (0800 hours) test.

  2. Discontinue all medications for 24–48 hours before the study. Especially important are spironolactone, estrogens, birth control pills, cortisol, tetracycline, stilbestrol, and phenytoin. Check with the healthcare provider.

  3. Weigh the patient and record weight.

  4. Have baseline blood cortisol drawn at 8:00 a.m. (0800 hours) and 4:00 p.m. (1600 hours). Give 1 mg dexamethasone at 11:00 p.m. (2300 hours) the same day. Draw blood at 8:00 a.m. (0800 hours) the next morning.

  5. Ensure that no radioisotopes are administered within 1 week before test.

  6. Follow guidelines in Chapter 1 regarding safe, effective, informed pretest care.

Posttest Patient Care

  1. Have patient resume normal activities.

  2. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel appropriately for Cushing syndrome or depression.

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

Interfering Factors ⬆

False suppression can occur in the following conditions:

  1. Pregnancy

  2. High doses of estrogens

  3. Alcoholism

  4. Uncontrolled diabetes

  5. Trauma, high stress, fever, dehydration

  6. Phenytoin (see Appendix E for other drugs)