What initial tests should I perform to confirm a diagnosis of Conn's syndrome in a patient with hypertension?

Initial tests to confirm a diagnosis of Conn's syndrome (primary hyperaldosteronism) in a hypertensive patient include measurement of the plasma aldosterone concentration (PAC) and plasma renin activity (PRA) or direct renin concentration (DRC) to calculate the aldosterone-to-renin ratio (ARR). This screening test is the cornerstone for detecting inappropriate aldosterone secretion causing hypertension NICE NG136 Funder et al. 2016. Blood samples should be taken after correcting hypokalaemia if present, and ideally when the patient is not on interfering medications such as mineralocorticoid receptor antagonists, ACE inhibitors, ARBs, or diuretics, as these can affect the ARR. Confirming an elevated ARR with a high aldosterone level and suppressed renin supports the diagnosis of Conn's syndrome NICE NG136 Sabbadin & Fallo 2016.

Following a positive ARR, confirmatory testing such as saline infusion test, oral sodium loading test, or fludrocortisone suppression test is recommended to establish autonomous aldosterone secretion. However, the initial step remains the ARR screening in a hypertensive patient suspected of primary aldosteronism NICE NG136 Araujo-Castro 2022. Additional baseline blood tests include serum potassium measurement, as hypokalaemia is a common but not universal feature.

In summary, the initial diagnostic approach in primary care or general practice for suspected Conn's syndrome is to perform plasma aldosterone and renin measurements to calculate the ARR, alongside serum potassium, before referral for confirmatory testing and imaging. This approach aligns with UK NICE guidance and is supported by international endocrine society recommendations NICE NG136 Funder et al. 2016Sabbadin & Fallo 2016Araujo-Castro 2022.