Abstract Context Primary aldosteronism (PA) screening relies on an elevated aldosterone to renin ratio with a minimum aldosterone level, which varies from 10 to 15 ng/dL (277-415.5 pmol/L) using immunoassay. Objective To evaluate intra-individual coefficient of variation (CV) of aldosterone and aldosterone to direct renin concentration ratio (A/DRC) and its impact on PA screening. Methods A total of 671 aldosterone and DRC measurements were performed by the same chemiluminescence assays in a large cohort of 216 patients with confirmed PA and at least 2 screenings. Results The median intra-individual CV of aldosterone and A/DRC was 26.8% and 26.7%. Almost 40% of the patients had at least one aldosterone level <15 ng/dL, 19.9% had at least 2 aldosterone levels <15 ng/dL, and 16.2% had mean aldosterone levels <15 ng/dL. A lower cutoff of 10 ng/dL was associated with false negative rates for PA screening of 14.3% for a single aldosterone measurement, 4.6% for 2 aldosterone measurements, and only 2.3% for mean aldosterone levels. Considering the minimum aldosterone, true positive rate of aldosterone thresholds was 85.7% for 10 ng/dL and 61.6% for 15 ng/dL. An A/DRC >2 ng/dL/µIU/mL had a true positive rate for PA diagnosis of 94.4% and 98.4% when based on 1 or 2 assessments, respectively. CV of aldosterone and A/DRC were not affected by sex, use of interfering antihypertensive medications, PA lateralization, hypokalemia, age, and number of hormone measurements. Conclusion Aldosterone concentrations had a high CV in PA patients, which results in an elevated rate of false negatives in a single screening for PA. Therefore, PA screening should be based on at least 2 screenings with concomitant aldosterone and renin measurements.