What are the typical plasma renin activity, plasma aldosterone concentration, and aldosterone-to-renin ratio in primary hyperaldosteronism (Conn’s syndrome)?

Conn's Syndrome: Plasma Renin and Aldosterone Ratio

Typical Laboratory Findings in Primary Aldosteronism (Conn's Syndrome)

In Conn's syndrome, plasma aldosterone concentration is elevated (≥10-15 ng/dL), plasma renin activity is suppressed (often <0.5 ng/mL/h), and the aldosterone-to-renin ratio (ARR) is markedly elevated (≥20-30). 1

Plasma Aldosterone Concentration (PAC)

• Elevated aldosterone: PAC must be at least 10-15 ng/dL to interpret a positive screening test, though many patients with confirmed primary aldosteronism have levels well above this threshold. 1
• In confirmed cases of Conn's syndrome, aldosterone levels typically range from 200 pg/mL or higher, with some patients reaching levels exceeding 500 pg/mL. 2
• The aldosterone elevation is autonomous and cannot be suppressed with sodium loading, which distinguishes it from physiologic aldosterone secretion. 1

Plasma Renin Activity (PRA)

• Suppressed renin: PRA is characteristically low or undetectable due to aldosterone-induced sodium retention and mild extracellular volume expansion. 3
• The specificity of ARR improves when a minimum PRA of 0.5 ng/mL/h is used in calculations, as extremely low renin values can create falsely elevated ratios. 1
• Important caveat: In rare cases of severe hypertensive kidney damage secondary to longstanding primary aldosteronism, PRA may escape suppression and appear normal or even elevated, though the ARR remains elevated due to disproportionately high aldosterone levels. 4

Aldosterone-to-Renin Ratio (ARR)

Calculation and Interpretation

• ARR = Plasma Aldosterone Concentration (ng/dL) ÷ Plasma Renin Activity (ng/mL/h) 1
• A positive screening test requires ARR ≥20-30 (most guidelines use ≥30 as the threshold) AND plasma aldosterone ≥10-15 ng/dL. 1, 5
• When using plasma renin concentration instead of activity, the threshold is approximately ARR ≥150 pmol/ng (or ≥50 pg/mL per pg/mL when using different units). 2, 6

Diagnostic Performance

• In confirmed Conn's syndrome, ARR values typically range from 50 to >500, with most patients having ratios well above the screening threshold. 2
• Using an ARR cutoff of ≥50 provides 89% sensitivity and 96% specificity for primary aldosteronism when patients are on ongoing antihypertensive medications. 2
• Specificity increases to 100% when combining ARR ≥50 with aldosterone ≥200 pg/mL, though sensitivity decreases slightly to 84%. 2

Critical Testing Considerations

Patient Preparation

• Potassium repletion is mandatory before testing, as hypokalemia suppresses aldosterone production and causes false-negative results; target serum potassium 4.0-5.0 mEq/L. 1
• Patients should have unrestricted salt intake before testing to avoid false-positive results from volume depletion. 1
• Blood should be collected in the morning (0800-1000 hours) with the patient out of bed for 2 hours and seated for 5-15 minutes immediately before collection. 1

Medication Interference

• Mineralocorticoid receptor antagonists (spironolactone, eplerenone) must be withdrawn for at least 4 weeks before testing. 1
• Beta-blockers and diuretics should be stopped when feasible, as they suppress renin and cause false-positive ARR results. 1
• ACE inhibitors and ARBs increase renin and cause false-negative ARR results but may be continued if stopping them is unsafe; interpret results in context. 1
• Long-acting calcium channel blockers and alpha-receptor antagonists minimally interfere with ARR and are safe alternatives during the washout period. 1

Confirmatory Testing Required

• A positive ARR is only a screening test and requires confirmatory testing to demonstrate autonomous aldosterone secretion that cannot be suppressed with sodium loading. 1
• Confirmatory options include intravenous saline suppression test (failure to suppress PAC below 5 ng/dL after 2L normal saline over 4 hours) or oral sodium loading with 24-hour urine aldosterone measurement. 1

Common Diagnostic Pitfalls

• Do not rely on hypokalemia to trigger screening, as it is absent in approximately 50% of primary aldosteronism cases. 1, 7
• Do not skip confirmatory testing based on ARR alone, as false-positives occur commonly due to low-renin states from volume expansion or dietary salt. 1
• In patients with severe hypertension and renal impairment, PRA may not be fully suppressed despite true primary aldosteronism due to secondary hypertensive kidney damage. 4