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rone metabolites in the plasma of these patients. The effect of this is that a commonly used automated aldosterone immunoassay overestimates plasma aldosterone in renal impairment, and the magnitude of this bias increases with the worsening of renal function. In comparison to results for individuals with normal renal function, the assay overestimates aldosterone on average by 129% in hemodialysis patients and 42% in moderate renal failure [eGFR 30–60 mL min 1 (1.73 m) ] and 27% in patients with mild renal impairment [eGFR 60–90 mL min 1 (1.73 m) ]. Overestimation of aldosterone in patients on dialysis and in endstage renal failure is not widely recognized although it has been reported previously (1–3). Our study is the first to demonstrate that this problem extends to mild and moderate renal impairment. Although large differences between immunoassays for aldosterone have previously been described (4), it is likely that direct immunoassays for plasma aldosterone other than the one we used would also be subject to interference from aldosterone metabolites in patients with renal impairment. The screening and diagnosis of diseases such as primary aldosteronism depend on the accuracy of aldosterone measurements. Up to one-fifth of patients with newly diagnosed primary aldosteronism may have significant renal impairment (5) and the assay overestimation we describe may increase the likelihood of incorrect diagnosis at both the screening and confirmatory stages of investigation of such patients. We note, however, that the clinical outcomes reported in the existing literature are primarily based on aldosterone immunoassays. Although we have not confirmed this finding, the requirement of solvent extraction in mass spectrometry would eliminate the overestimation observed. Indeed, aldosterone is not overestimated by mass spectrometry in patients on hemodialysis (2). Our results indicate that caution is needed in the interpretation of plasma aldosterone measurements by immunoassay in patients with renal impairment, and, when accurate measurements are required, a liquid extraction step or mass spectrometry should be considered.