Which test is most useful in evaluating a patient with suspected cushing syndrome?

The 24-hour urine cortisol test is a highly effective tool for evaluating suspected Cushing syndrome because it directly measures the total cortisol output by the adrenal glands over a full day. This method provides a comprehensive assessment of cortisol secretion and is particularly useful since Cushing syndrome is characterized by abnormal cortisol levels. In patients suspected of having Cushing syndrome, elevated levels of cortisol in the urine indicate excessive production, which can confirm the diagnosis.

By collecting urine over a 24-hour period, fluctuations in cortisol levels throughout the day are averaged out, making this test more reliable than single-point measurements. Elevated urinary free cortisol may also help differentiate Cushing syndrome from other disorders related to cortisol metabolism. While other tests like the dexamethasone suppression tests and plasma ACTH levels can provide additional information in the diagnostic process, the 24-hour urine cortisol test stands out as the most definitive initial screening method due to its ability to reflect the overall cortisol production accurately.