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Primary aldosteronism (PA) is associated with left ventricular hypertrophy and atrial fibrillation; however, there are few data on its adverse renal effects.To establish whether PA is associated with greater renal damage in patients with normal glomerular filtration rate (GFR) than is primary hypertension.The prospective, multicenter, Primary Aldosteronism Prevalence in Italy (PAPY) study consecutively recruited individuals who had been referred for specialist treatment after a new diagnosis of primary hypertension. Those with a serum creatinine level <115 μmol/l (<1.3 mg/dl) and creatinine clearance >61 ml/min/1.73 m2 (as estimated by the abbreviated MDRD equation) were eligible to enroll. Diabetes, congestive heart failure and a history of secondary hypertension were among the exclusion criteria. Mineralocorticoid receptor antagonists were withheld for ≥6 weeks, and diuretics, β-blockers, angiotensin-converting-enzyme inhibitors and angiotensin receptor blockers for ≥2 weeks. The presence and etiology of PA was assessed by two investigators, on the basis of the aldosterone:renin ratio before and after administration of 50 mg captopril, and calculation of a multivariate logistic discriminant function. If the findings were positive for PA, a saline infusion test confirmed or excluded the diagnosis. Imaging tests, adrenal vein sampling and adrenocortical scintigraphy were used to aid diagnosis of aldosterone-producing adenoma. GFR and urine albumin excretion (UAE) rate were recorded at baseline.The primary endpoints were 24 h UAE and GFR (as estimated by the abbreviated MDRD equation).Complete UAE and GFR data were available for 490 patients. Of the 64 (13.1%) who had PA, 33 (mean age 49 years) had idiopathic hyperaldosteronism and 31 (mean age 51.5 years) had aldosterone-producing adenoma. The remaining 426 patients (mean age 45 years) had primary hypertension. Age, plasma aldosterone level and BMI predicted GFR in the stepwise regression analysis; age, BMI, mean blood pressure, and serum potassium level were predictors of UAE rate. After adjustment for these variables, patients with PA had a significantly greater mean level of albuminuria than patients with primary hypertension (28.1 μg/ml vs 18.8 μg/ml; P<0.001). The prevalence of microalbuminuria was also greater in the PA group than in the primary hypertension group (χ2=9.92; P=0.002). Mean adjusted GFR was slightly higher among the patients with primary hypertension than among those with aldosterone-producing adenoma or idiopathic hyperaldosteronism (92 ml/min/1.73 m2 vs 85 ml/min/1.73 m2 and 88 ml/min/1.73 m2, respectively). Overall, there was no difference in the level of UAE or the frequency of microalbuminuria between patients with hypokalemia (serum potassium <3.5 mmol/l) and those with normokalemia.In patients without overt renal dysfunction, PA is associated with greater renal damage than is primary hypertension.