Introduction: Accurate pre-operative imaging of a parathyroid adenoma facilitates minimally invasive surgery for primary hyperparathyroidism, increases cure rate and reduces perioperative complications. The two most commonly deployed preoperative localization studies are ultrasonography (US) and parathyroid scintigraphy using 99m-techentium sestamibi (RN). Patients who have concordant results between the two studies (most studies report concordance rate of 60%) usually undergo minimally invasive surgery whilst those with non-concordance scan results often need bilateral open neck surgery.Objectives: We did a retrospective assessment of the clinical, biochemical and imaging parameters of patients who had parathyroid surgery in our hospital to assess (a) the sensitivity and positive predictive value (PPV) of US and RN scans (b) the frequency of concordance between the two imaging studies (c) the clinical and laboratory predictors of concordance and (d) the impact of concordance on the cure rate.Method: 155 patients who were operated for PHPT between January 2011 and January 2019 were included. All patients underwent preoperative localization with US and RN imaging. The sensitivity and PPV of the 2 imaging procedures in detecting a parathyroid adenoma were determined by correlating the imaging findings of both scans with the composite information obtained from surgical findings and post-operative biochemical results to indicate cure. The patients with concordant and non-concordant imaging findings were compared for surgical cure rate, serum calcium and parathormone level, and the volume and weight of the adenoma.Results: The sensitivity and PPV of US were 80.9% and 82.8% and for RN scan 78.7% and 87.8% respectively. There was no statistically significant difference in the accuracy between the two modalities. 93(60%) patients had concordant and 62(40%) patients had non-concordant scan results, which included true discordance and non-localization by one or both scans. Cure rate in concordant and non-concordant scans were 96.8% and 83.7% respectively (p=0.02, chi-square). In comparison to patients with non-concordant imaging, patients with concordant imaging had higher level of serum calcium (mean 3.02 vs 2.86, p=0.04), the resected adenoma was larger in volume (mean 3109mm3 vs 2083mm3, p=0.05) and was heavier (mean 1.59 vs 1.10 p=ns). However there was no difference in the age or serum PTH level between the 2 groups of patients.Conclusion: Both US and RN imaging have similarly high sensitivity and PPV in identifying a parathyroid adenoma and our figures were comparable to the published literature. When the two studies are concordant a significantly higher surgical success rate is obtained. Patients with higher serum calcium and larger adenomas are more likely to demonstrate concordant imaging.