Will a saliva cortisol test be accurate 1-2 weeks after a Dexamethasone (Dex) suppression test?

From the Guidelines

A saliva cortisol test should not be performed 1-2 weeks after a dexamethasone (Dex) suppression test, as the results may be inaccurate due to the lingering effects of dexamethasone on cortisol production. The half-life of dexamethasone is approximately 36-54 hours, but its suppressive effect on the hypothalamic-pituitary-adrenal (HPA) axis can persist longer, potentially causing falsely low cortisol readings in subsequent tests 1.

Key Considerations

• Dexamethasone is a synthetic glucocorticoid that suppresses cortisol production by acting on the HPA axis.
• The suppressive effect of dexamethasone on the HPA axis can last for several weeks after administration.
• Measuring dexamethasone levels along with cortisol can improve test interpretability, but this may not be feasible in all cases 1.
• False positive results may be seen with rapid absorption or malabsorption of dexamethasone, concomitant treatment with CYP3A4 inducers, or increased corticosteroid binding globulin (CBG) levels 1.

Recommendations

• Wait at least 2-4 weeks after a Dex suppression test before conducting a saliva cortisol test to ensure accurate results.
• If cortisol testing is necessary sooner, consult with a healthcare provider about alternative assessment methods or how to interpret potentially affected results.
• Always inform providers ordering endocrine tests about recent dexamethasone exposure to ensure proper interpretation of results.

From the Research

Accuracy of Saliva Cortisol Test after Dexamethasone Suppression Test

• The accuracy of a saliva cortisol test 1-2 weeks after a Dexamethasone (Dex) suppression test is a topic of interest in endocrinology.
• A study from 1998 2 found that the reproducibility of the low-dose dexamethasone suppression test was lower when using salivary cortisol measurements compared to plasma cortisol measurements.
• The study concluded that salivary cortisol measurements may only be appropriate for large numbers of subjects in field studies of dexamethasone suppression.

Factors Affecting Test Accuracy

• Other studies have investigated factors that may affect the accuracy of the dexamethasone suppression test, such as oral contraceptive use 3, 4.
• These studies found that oral contraceptive use can lead to false-positive results and that the test may need to be repeated after withdrawal of the contraceptive.
• The use of free cortisol assessment has also been shown to enhance the interpretation of abnormal dexamethasone suppression test results 4.

Salivary Cortisol Measurement

• A study from 2003 5 found that salivary cortisol measurement presented a higher percentage of suppression than plasma cortisol after dexamethasone suppression test in patients with Cushing's disease.
• The study suggested that measurement of salivary cortisol might improve the dexamethasone suppression test in the differential diagnosis of Cushing's syndrome.
• However, the study did not specifically address the accuracy of salivary cortisol tests 1-2 weeks after a dexamethasone suppression test.

Relevance to the Question

• Based on the available evidence, it appears that the accuracy of a saliva cortisol test 1-2 weeks after a dexamethasone suppression test may be affected by various factors, including the reproducibility of the test and the use of oral contraceptives.
• However, there is limited direct evidence to answer the question, and more research may be needed to fully understand the accuracy of salivary cortisol tests in this context 2, 3, 4, 5.