OBJECTIVE: Plasma aldosterone concentration (PAC) to plasma renin activity (PRA) ratio is an established screening test for primary hyperaldosteronism. Due to the increased recognition of adrenal incidentalomas, reliable parameters are required. Determination of active renin concentration (ARC) in contrast to PRA offers advantages with regard to processing and standardization. The present study compared PRA and ARC under random conditions to establish thresholds for the diagnosis of primary hyperaldosteronism. DESIGN AND METHODS: Fifty patients with various adrenal tumors, including ten patients with aldosterone-secreting adrenal adenomas, as well as ten hypertensive patients and 23 normotensive volunteers were studied. PAC and PRA were measured by radioimmunoassay. ARC was determined by an immunoluminometric assay. RESULTS: Receiver operating curve (ROC) analysis suggested a PAC to ARC ratio threshold of 90 ((ng/l)/(ng/l)) (sensitivity 100%, specificity 98.6%) and a ratio threshold of 62 by additional consideration of PAC > or =200 ng/l (sensitivity 100%, specificity 100%) for the diagnosis of aldosterone-secreting adrenal adenomas. CONCLUSIONS: A PAC to ARC ratio of > or =62 in patients with PAC levels > or =200 ng/l is a reliable screening method for primary hyperaldosteronism in patients with an aldosterone-producing adenoma under random conditions. Because of its advantages with regard to probe processing and its independence from endogenous angiotensinogen levels, ARC may be preferred to PRA.