Endocrine Abstracts

Midnight Cortisol Revisited

D Noimark, MT Barakat, K. Meeran, JF Todd

Endocrine Unit, Imperial College School of Medicine, Hammersmith Hospital, Du Cane Road, London

Background: The diagnosis of Cushing's syndrome must be established before any attempt at differential diagnosis. The diagnosis of Cushing's syndrome can be made initially on an outpatient basis with the measurement of 24 hour urinary free cortisol and an overnight dexamethasone suppression test. However, this latter test is associated with significant false positive and negative results. Inpatient testing include midnight and 9am plasma cortisol levels as patients with Cushing's lose the normal circadian rhythm. The low dose dexamethasone test (LDDST) remains the gold standard for screening for Cushing's syndrome although all tests are useful and sometimes essential adjunctives.

Aim: We reviewed the use of midnight cortisol measurements in 18 patients we have investigated with Cushing's syndrome to try and establish whether it offered any additional information.

Results:

All the patients diagnosed with Cushing's syndrome had elevated midnight cortisols. In two patients the midnight cortisol value gave a false positive result and appear discrepant from the LDDST result. The midnight cortisol has a sensitivity of 100% (10/10) and a specificity of 75% (6/8). Therefore, this test has a negative predictive value of 100% (6/6) and a positive predictive value of 83 % (10/12).

Conclusion: The midnight cortisol may be useful in excluding Cushing's syndrome as a low midnight cortisol has a negative predictive value of 100%. However, an elevated midnight cortisol occurs in both patients with Cushing's syndrome and some normal patients. Therefore a low midnight cortisol excludes Cushing's syndrome but a raised level warrants further investigations. We would recommend its use in all patients who are being investigated for Cushing's syndrome.