Although most patients have essential (unexplained) hypertension, some patients have a treatable underlying condition. One such condition is phaeochromocytoma, a tumour that produces excessive stress hormones. Left undiagnosed, patients may develop a hypertensive crisis that can be fatal. Measurements of stress hormones (both 24-hour urine collection and morning blood tests) are highly sensitive for detecting these tumours. However, these stress hormones may also be elevated in obstructive sleep apnoea (OSA) which affects 1 in 5 adults.

The investigators hypothesize that in patients with OSA, blood tests will be better than 24-hr urine tests at ruling out a tumour. If this is confirmed, then OSA patients with suspected phaeochromocytoma could be investigated with a morning blood test instead of a traditional urine test, reducing unnecessary additional tests and patient anxiety.

In this single site study, the investigators plan to recruit 70 patients undergoing polysomnography. 24hr urine and bloods will be measured. Patients with elevated hormone levels will undergo imaging to rule out a tumour.

The primary outcome will be the accuracy of each test in ruling out a tumour. The secondary outcomes will be the relationship between stress hormone level and severity of OSA, which may help to explain the increased cardiovascular risk in patients with OSA, and the change in stress hormone level with treatment for OSA

Inclusion Criteria:

• Patients aged 21-70 years attending for a sleep study to evaluate for OSA

Exclusion Criteria:

• Medications that can cause changes in metanephrine levels
• Other factors that can cause changes in metanephrine levels
• Renal impairment (eGFR < 60ml/min)
• Other serious medical conditions

Patients attending for sleep studies for evaluation of OSA