Which laboratory test is used to screen for primary aldosteronism?

The aldosterone to renin ratio is the primary laboratory test used to screen for primary aldosteronism, also known as Conn's syndrome. This condition is characterized by excess production of aldosterone, which can lead to hypertension and hypokalemia.

The rationale for using this test lies in its ability to assess the balance between aldosterone and renin, two key hormones that regulate blood pressure and fluid balance. In primary aldosteronism, there is usually increased aldosterone production with suppressed renin levels due to the negative feedback mechanism. As a result, an elevated aldosterone to renin ratio serves as a strong indicator of the presence of this disorder.

The other tests mentioned would not be specific for primary aldosteronism. Plasma metanephrines are primarily used to screen for pheochromocytoma, a different adrenal condition. Midnight salivary cortisol is typically utilized for diagnosing Cushing's syndrome by measuring cortisol levels at a time when they should be low. Similarly, the 24-hour urinary free cortisol test is used to evaluate cortisol levels, again more relevant for Cushing's syndrome rather than aldosteronism. Thus, the aldosterone to renin ratio is the most appropriate and relevant test for screening primary aldosteron