In the medical world, there are many health conditions that show proper characteristic symptoms like blood-stained sputum in case of haemoptysis or reddening of hands in case of palmar erythema. These signs mainly help the doctor diagnose the condition at quite an early stage. However, there are also some health issues like Cushing's Syndrome that are really difficult to understand as the characteristic signs and symptoms mimic many other diseases.

When a person has Cushing syndrome, usually the body has high levels of cortisol, a hormone that is made by the adrenal glands. It mainly affects everything from blood pressure to memory. The problem with Cushing's Syndrome is that the signs can look a lot like many other diseases which mainly leads doctors to do more than one test to make sure the person has the condition. One of those tests that help to diagnose Cushing’s syndrome by determining the cortisol levels is an “Overnight Dexamethasone Suppression Test”.

What Is Overnight Dexamethasone Suppression Test?

Overnight Dexamethasone suppression test is a type of diagnostic assay that chiefly measures the levels of cortisol within the body and whether the adrenocorticotrophic hormone (ACTH) secretion by the pituitary gland can be suppressed. It is a test that mainly helps to diagnose Cushing Syndrome.

What Is Cushing Syndrome?

Cushing Syndrome is a type of health condition when you have high levels of cortisol for a long time. The high cortisol levels can typically cause weight gain, thinning skin, easy bruising, and other issues. Without treatment for a long time, it can lead to conditions such as type 2 diabetes, high blood pressure and even bone loss (osteoporosis).

Also Read: Cushing Syndrome: Causes, Symptoms And Treatment

High levels of cortisol usually happens if there is a tumour in the pituitary gland. Although most tumours are non-cancerous, some are malignant in nature. A tumour on your pituitary gland is one of the most common one causes of Cushing syndrome but it is usually benign. It usually triggers the pituitary gland to make too much adrenocorticotropic hormone (ACTH), which in turn stimulates the  adrenal glands to produce large and unregulated amounts of cortisol. Cushing syndrome also occurs due to adrenal tumours but they can also be benign or malignant in nature.

Also Read: Cortisol: Structure, Crucial Functions, Adverse Effects

Who Needs The Overnight Dexamethasone Suppression Test?

The doctor usually suggests this test if the patient shows signs and symptoms of Cushing syndrome, like getting bruised easily, gaining weight around the belly, and a very round face. The doctor might also conduct this test if the person has problems that aren’t typical for his or her age but could mean that they might have Cushing syndrome: For example, if the person is young but has weak bones (osteoporosis), high blood pressure, or diabetes.

Why Is The Test  Performed?

This test is primarily done when the doctor suspects that the patient’s body is producing too much cortisol. It is done to help diagnose Cushing syndrome and identify the specific causative factor. It can be done by giving two types of doses. While the low-dose test can help tell whether the patient’s body is producing too much ACTH, the high-dose test usually helps determine whether the problem is in the pituitary gland (Cushing disease) or from a different site in the body (ectopic).

How To Prepare For The Test?

The Overnight Dexamethasone suppression test usually requires avoiding food and any drink for about 10-12 hours before the morning blood test . The person going for the test must also inform the doctor if he or she is taking antibiotics, anti-seizure medications, medicines that contain corticosteroids, such as hydrocortisone, prednisone, estrogen pills, oral birth control pills (contraceptives), water pills (diuretics) or the person is pregnant. In such cases, the test results are affected hence the doctor might suggest some other tests to measure the cortisol levels.

How Is The Test Performed?

During this test, the person usually receives dexamethasone. This is a strong man-made (synthetic) glucocorticosteroid that binds to the same receptor as cortisol. Dexamethasone mainly reduces ACTH release in normal people. Therefore, taking dexamethasone should reduce ACTH level and lead to a decreased cortisol level. Next, blood is drawn out so that the cortisol level in the blood can be measured.

Dexamethasone suppression test is usually of two types: Low dose and High dose, each of which can either be done in an overnight, which is common or standard (3-day) method, which is quite rare.

Common:

Low-Dose overnight – The person gets 1 milligram (mg) of dexamethasone at 11 p.m., and the phlebotomist usually draws out blood the next morning at 8 a.m. for the cortisol measurement.

High-Dose Overnight – The phlebotomist first measures the cortisol level on the morning of the test. Then the person receives 8 mg of dexamethasone at 11 p.m. and the next morning around 8 a m. blood is drawn out for the cortisol measurement.

Rare:

Standard Low-Dose –  In this test, instead of blood urine is collected over 3 days (stored in 24-hour collection containers) from the person going for the test to measure cortisol. On day 2, the person gets a low dose (0.5 mg) of dexamethasone by mouth every 6 hours for 48 hours.

Standard High-dose – In this test, urine is collected over 3 days (stored in 24-hour collection containers) from the person going for the test to measure cortisol. On day 2, the person gets a high dose (2 mg) of dexamethasone by mouth every 6 hours for 48 hours.

Are There Any Side Effects Of The Test?

There are usually no reported side effects from taking an Overnight Dexamethasone suppression test apart from slight pain and bruising at the puncture site that too only for some time after which it usually subsides on application of an ice pack or on its own. In very rare case scenarios, there were reports of infection at the puncture site, hematoma, multiple punctures to locate vein, light headedness, excessive bleeding, or dizziness.

How To Interpret The Test Results?

Since results usually vary depending upon the labs, so the person undergoing the test must talk to the doctor to learn exactly what the numbers mean and what should be the next treatment strategy.

For a low-dose test, a healthy cortisol level is mainly below 1.8 mcg/dL (micrograms per deciliter) or 50 nmol/L (nanomoles per liter). If the test result is higher than that, there’s a good chance that the person is suffering from Cushing's syndrome. If it’s lower, the high cortisol levels might be getting triggered by something else which requires other test procedures.

For a high-dose test, the doctor mainly looks for a 50% drop in the cortisol level. If the level drops, it’s likely that a pituitary tumour might be causing Cushing's syndrome, whereas if the level doesn’t drop, the person might have a tumour somewhere else in the body.