"Dexamethasone-suppression
test" (DST) helps to illuminate neuroendocrine
abnormalities. Many depressive patients (about 50%) produce abnormally large
amount of cortisol, although it is not clear whether this is a cause or an
effect of the illness. These patients fail to suppress the
hypothalamic-pituitary-adrenal axis when taking dexamethasone, and DST test is
abnormal. This test can be useful in clarifying the diagnosis between
schizophrenia and a depression with many psychotic features. This test does
seem to suggest that part of the cause of the illness may reside in the
hypothalamus or the higher brain centers that govern it.
The DST was the first, and is to date the most
studied, putative biological marker in research on depressive disorders. In
1968 Carroll showed that while depressed, patients fail to suppress plasma
cortisol. This led Carroll to claim that a positive DST is a specific laboratory
test for melancholia or severe forms of depression.
The sensitivity of the DST (rate of a positive
outcome, or nonsuppression of cortisol) in major depression is modest (about
40%-50%) but is higher (about 60%-70%) in very severe, especially psychotic,
affective disorders, including major depression with psychotic as well as
melancholic features, mania, and schizoaffective disorder. The specificity
(true negative outcome) of the DST in normal control subjects is above 90%, but
it varies from less than 70% to more than 90% in psychiatric conditions that
often need to be separated from major affective disorders. In dementia the
specificity is even lower.
In addition, a number of medical conditions,
including severe weight loss and use of alcohol and certain other drugs
(barbiturates, anticonvulsants, and others), can produce false positive
results. Positive initial DST status in major depression does not add
significantly to the likelihood of antidepressant response, and a negative test
is not an indication for withholding antidepressant treatment. Some recent data
suggest that DST-positive depressions (cortisol nonsuppression) are less likely
than DST-negative cases (cortisol suppression) to respond to a placebo. If this
is confirmed, it would increase the real magnitude of the difference in
treatment response between DST-positive and DST-negative depressed patients.
Failure to convert to normal suppression of cortisol with apparent recovery
from depression suggests an increased risk for relapse into depression or
suicidal behavior. Although the clinical utility of the DST as currently
understood is limited, in certain specific situations its thoughtful use may
aid clinical decision making. The association of an abnormal test result with
major affective disorders encourages continued research on the DST.
Procedure
The test is done
by giving the patient with suspected depressive illness a once-off low dose (1
mg) of dexamethasone (a steroid medication) about 11 pm one night, and taking
the blood for testing late the next afternoon or evening. It is
recommended to take the pill with
milk or an antacid to help prevent an upset stomach or heartburn. Remember that
you will not be able to eat or drink anything for 10 to 12 hours before the
morning blood test.
Note that some medicines
may change the results of this test. Be sure to tell your doctor about all the
nonprescription and prescription medicines you take. You may be asked to stop
taking some medicines (such as birth control pills, aspirin, morphine,
methadone, lithium, monoamine oxidase inhibitors [MAOIs], and diuretics) for 24
to 48 hours before your blood is drawn. Talk to your doctor about any concerns
you have about the need for the test, its risks, how it will be done, or what
the results will mean.
The level of
cortisol (cortisone) in the blood is measured. The person without
depression responds to the presence of dexamethasone in their body by
drastically reducing (suppressing) the amount of cortisol in their blood
automatically, so their blood test shows low levels of cortisol. However,
in about 50 - 60% of people with depressive illness, their body ignores the
presence of dexamethasone, and this blood test shows HIGH levels of cortisol
the next day. The failure to suppress cortisol production despite
dexamethasone confirms the diagnosis of depressive illness. Normal suppression
gives levels about 30-40 nmol/l. Levels above 140 after dexamethasone are
normally regarded as a positive test indicative of depressive illness, in the
absence of severe physical illness, enzyme inducing medications, high alcohol
intake or hormonal illnesses.
That is still
not clear why there are two different types of depressive illness, namely one
type which can be measured by this test, and another type (with identical
clinical symptoms) which cannot be measured in the laboratory. Nor that is
not clear why the DST is abnormal in some depressed patients while they are
ill, returning to normal when they recover.
Your own
physician or psychiatrist can discuss the benefits and limitations of this test
in your particular case. An abnormal DST is not enough evidence alone to
diagnose depressive illness (physically ill, malnourished and elderly people
have abnormal DST’s in about 20% of cases). However, in people who are
told their symptoms are due to unhappiness or due to unresolved emotional
hang-ups, an abnormal DST is very good reason to suggest they have gone on to
develop depressive illness, as a result of the stresses involved.
It must also be
remembered that 50% of people with depressive illness still DO have the
illness, although they respond normally on this test, just as some people have
back pain without any abnormality showing on their X-rays.
What Affects the Test?
- Pregnancy or extreme obesity.
- Severe weight loss, dehydration, or acute alcohol
withdrawal.
- Severe injury.
- Diabetes.
- You take medicines, such as barbiturates, phenytoin
(Dilantin), birth control pills, aspirin, morphine, methadone, lithium,
monoamine oxidase inhibitors (MAOIs), spironolactone (Aldactone), and diuretics.
Some people may
quickly process (metabolize) the dose of dexamethasone. In these people,
cortisol levels will not drop unless a higher dose of the medicine is given.
Sources
and Additional Information:
