Does a serum cortisol of 0.8 µg/dL the morning after taking 0.5 mg dexamethasone at night indicate adequate suppression and argue against Cushing’s syndrome?

Dexamethasone Suppression Test Interpretation

A serum cortisol of 0.8 µg/dL following 0.5 mg dexamethasone taken at 11 PM-midnight indicates adequate suppression and effectively rules out Cushing's syndrome. 1

Understanding Your Test Result

Your cortisol level of 0.8 µg/dL is well below all established diagnostic thresholds for Cushing's syndrome:

• The standard cutoff is <1.8 µg/dL (50 nmol/L) for adequate suppression, which your result clearly meets 2, 1
• Even using the more stringent cutoff of <2.1 µg/dL (which has 95% sensitivity and 93% specificity), your result indicates normal suppression 3
• The most conservative threshold ever suggested is <5 µg/dL, and your result is far below this 4

Why This Matters Clinically

Your result argues strongly against Cushing's syndrome because:

• The 1 mg overnight dexamethasone suppression test has >90% sensitivity for detecting Cushing's syndrome when negative 1
• In patients with confirmed Cushing's disease, only 8% suppress cortisol to <2 µg/dL, and your result is even lower than this rare false-negative threshold 4
• Studies show virtually no false-negative results at cortisol levels as low as yours 5

Important Caveat About Your Dose

You took 0.5 mg dexamethasone instead of the standard 1 mg dose 1. This is a critical detail:

• The lower dose could theoretically reduce suppression effectiveness, though your result is so profoundly suppressed (0.8 µg/dL) that this is unlikely to be clinically relevant 1
• If there remains clinical suspicion despite this result, repeat the test with the standard 1 mg dose to ensure adequate dexamethasone exposure 1
• Measuring serum dexamethasone levels can confirm adequate drug absorption if results are equivocal, though unnecessary in your case given the profound suppression 3

When to Pursue Further Testing

Further evaluation would only be warranted if:

• Strong clinical features of Cushing's syndrome persist (facial plethora, easy bruising, purple striae >1 cm, proximal muscle weakness) 6, 7
• Cyclic Cushing's syndrome is suspected, which requires repeated testing during symptomatic periods 1
• The test is repeated with 1 mg dexamethasone and shows inadequate suppression 1

Your result of 0.8 µg/dL provides strong reassurance against autonomous cortisol production, even with the lower dexamethasone dose used.