BACKGROUND: Adrenal vein sampling (AVS) is the gold standard for subtyping primary aldosteronism (PA), but the role of adrenocorticotropic hormone (ACTH) stimulation during this procedure remains debated. AVS is technically challenging and is available at limited centres; strategies that limit its use to selected patient groups are needed. METHODS: We retrospectively analysed 98 patients with biochemically confirmed PA who underwent AVS between 2018 and 2025. Outcomes were compared between unstimulated (n = 31) and ACTH-stimulated (n = 67) protocols, including bilateral cannulation success, lateralization rates, and concordance with adrenal CT. Correlations between biochemical markers and the lateralization index (LI) were assessed, and composite clinical scores were validated and developed to predict unilateral disease. RESULTS: Bilateral successful cannulation was achieved in 73.4% cases, with comparable rates between unstimulated and stimulated AVS (71.0% vs. 74.6%; p = 0.806). Lateralization was significantly higher in unstimulated AVS (68.2% vs. 34.0%; p = 0.027). CT-AVS concordance was low (44.9%). LI correlated positively with baseline plasma aldosterone (PAC), aldosterone-renin ratio (ARR), and post-saline PAC, and negatively with direct renin (DRC). Predictors of unilateral disease with sensitivity > 70% included PAC > 23.2 ng/dL, ARR > 17.7 ng/dL/mU/L and post-saline PAC > 8.38 ng/dL. The prediction score by Kobayashi et al. < 8 yielded sensitivity of 96.9% and specificity of 35% (AUC 0.712). Our composite score (PAC, ARR, male sex; threshold ≥ 20) achieved sensitivity of 81.5% and specificity of 65% (AUC 0.802). CONCLUSIONS: Unstimulated AVS provided superior lateralization without loss of cannulation success. Biochemical thresholds and clinical scoring may prioritise patients for AVS and simplify PA management algorithms.