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IMPORTANCE: Prior studies have shown that physicians in New York State (New York) perform twice as many cardiac catheterizations per capita as those in Ontario for stable patients. However, the role of patient selection in these findings and their implications for detection of obstructive coronary artery disease (CAD) are largely unknown. OBJECTIVE: To evaluate the extent of obstructive CAD and to compare the probability of detecting obstructive CAD for patients undergoing cardiac catheterization. DESIGN, SETTING, AND PATIENTS: An observational study was conducted involving patients without a history of cardiac disease who underwent elective cardiac catheterization between October 1,2008, and September 30,2011. Obstructive CAD was defined as diameter stenosis of 50% or more in the left main coronary artery or stenosis of 70% or more in a major epicardial vessel. MAIN OUTCOMES AND MEASURES: Observed rates and predicted probabilities of obstructive CAD. Predicted probabilities were estimated using logistic regression models. RESULTS: A total of 18 114 patients from New York and 54 933 from Ontario were included. The observed rate of obstructive CAD was significantly lower in New York at 30.4% (95% CI, 29.7%-31.0%) than in Ontario at 44.8% (95% CI, 44.4%-45.3%; P < .001). The percentage of patients with left main or 3-vessel CAD was also significantly lower in New York than in Ontario (7.0% [95% CI, 6.6%-7.3%] vs 13.0% [95% CI, 12.8%-13.3%]; P < .001). In New York, a substantially higher percentage of patients with low predicted probability of obstructive CAD underwent cardiac catheterization; for example, only 19.3% (95% CI, 18.7%-19.9%) of patients undergoing cardiac catheterization in New York had a greater than 50% predicted probability of having obstructive CAD than those in Ontario at 41% (95% CI, 40.6%-41.4%; P < .001). At 30 days, crude mortality for patients undergoing cardiac catheterization was slightly higher in New York at 0.65% (90 of 13 824; 95% CI, 0.51%-0.78%) than in Ontario at 0.38% (153 of 40 794; 95% CI, 0.32%-0.43%; P < .001). CONCLUSIONS AND RELEVANCE: In Ontario compared with New York State, patients undergoing elective cardiac catheterization were significantly more likely to have obstructive CAD. This appears to be related to a higher percentage of patients in New York with low predicted probability of CAD undergoing cardiac catheterization. [ABSTRACT FROM AUTHOR]