Ethnic minority patients represented 5% of the overall 5803 patients. They differed in cardiovascular risk profile from Western European patients, and although ethnic minority patients were much younger (55 vs 65 years), they had a higher prevalence of diabetes (27% vs 18%). Furthermore, they were less often female and more often current smokers. Treatment of multivessel disease was performed in 14% of ethnic minority and 19% of Western European patients. While this may suggest a somewhat lower disease burden in the trial participants with an ethnic minority background, the rate of patients with complex (i.e. calcified or bifurcated) target lesions did not differ between groups. The 1‑year rate of the main endpoint TVF did not differ between groups (3.5% vs 4.9%), and multivariate analysis with adjustment for confounders did not change this finding. In addition, there was no between-group difference in mortality, MI, and repeated revascularisation rates. While ethnic minority patients had a higher definite stent thrombosis rate (1.0% vs 0.3%), multivariate analysis revealed that ethnic background was not independently associated with stent thrombosis.

Earlier studies, assessing health disparities in countries with a Western lifestyle, have shown that ethnic minorities have an increased burden of cardiovascular risk factors such as diabetes and hypertension [13, 14]. The present study demonstrates that ethnic minority patients presenting with obstructive coronary artery disease were on average younger and had more risk factors, including diabetes, smoking, and a family history of coronary disease. This is in line with previous reports [15,16,17]. Our analysis shows that after adjustment for these potential confounding factors, in ethnic minority patients the event risk of our main clinical endpoint TVF was even lower (HR closer to 1.0). This suggests that not the ethnic background but the patient’s risk profile determines the incidence of adverse events. Despite differences in cardiovascular risk profile, 1‑year clinical outcome was similar in ethnic minority and Western European patients. Nevertheless, intensifying secondary prevention measures will be of great importance in preventing adverse events in the long term [18], as patients of various ethnic backgrounds can benefit from optimising cardiovascular risk profiles [19].

Most previous studies that assessed ethnical background in the context of coronary stenting were conducted in the eras of bare metal stents and first-generation DES. These studies suggested an increased adverse event risk in ethnic minority patients as compared to non-immigrants [7, 13]. Meanwhile, there has been substantial progress in DES design, PCI techniques, and concomitant medication [20, 21].

The present study in ethnic minority patients treated with new-generation DES did not show an elevated event risk, which is in line with a recent study from the United States [15]. That study pooled 4182 patients, treated with new-generation DES, from two observational studies and found no impact of ethnic background on 1‑year risks of MACE, mortality, and target vessel revascularisation. Yet, when the individual components of MACE were separately assessed, ethnic minorities encountered a higher risk of recurrent MI, primarily driven by non-stent-related MI [15]. This may reflect their less favourable cardiovascular risk profile. In addition, differences in health politics (e.g. general access to healthcare) may also account for a higher risk in patients with low socioeconomic status from the United States, who may have difficulties with medication adherence due to financial restrictions. In the present study, we did not encounter a higher risk of recurrent MI in the ethnic minority group.

Another analysis of pooled data from two observational studies, conducted in Europe and Asia, assessed potential differences between ethnic groups in risk profile and clinical outcome after PCI with polymer-free drug-coated stents [22]. The event rates in that study were similar to the event rates in our analysis with contemporary DES. Another study that evaluated use of a biodegradable polymer DES with anti-CD34+ antibody coating in Asian and European patients found a higher incidence of diabetes in Asian patients, similar to our ethnic minority population. In addition, our study findings are in line with their results that showed no short-term difference in adjusted clinical outcomes between Asian and European patients [23]. Furthermore, a nationwide cohort study in the United Kingdom assessed ethnic disparities in care and clinical outcome of patients who were treated with PCI or coronary artery bypass grafting for non-ST-segment elevation MI [17]. Ethnic minorities more often underwent repeated revascularisation, but they had rates of in-hospital mortality, MACE, and major bleeding that were similar to those of White patients.

In the present study, the ethnic minority patients had a higher incidence of definite stent thrombosis, but there was no independent association with ethnic background. This finding is in contrast to earlier studies from the United States, which found that ethnic (particularly African American) background was an independent risk factor for thrombus formation in first-generation DES [24]. Others explained inferior outcome in ethnic minority patients by different stent choices and the fact that African Americans were more likely to be treated with bare metal stents than White patients [25, 26]. Yet, the stent choice alone is unlikely to explain a higher rate of stent thrombosis.

In current clinical practice in the Netherlands and Belgium, discriminating stent use is highly unlikely, as DES implantation is the standard of care and new-generation DES are widely available [20, 21]. Likewise, in our present study, trial participants of all ethnic backgrounds were treated with randomly allocated new-generation DES, which precludes any stent choice. We can only speculate that our observation of a higher definite stent thrombosis rate in the unadjusted analysis is likely related to the higher cardiovascular risk profile that we observed in ethnic minority patients. The fact that the association between stent thrombosis and ethnic minority status was no longer significant after multivariate adjustment supports this notion. However, the overall incidence of stent thrombosis was low, and these results should be interpreted with caution.

All findings should be considered hypothesis generating. As the number of patients with an ethnic minority background was modest, we pooled data from two similar trials. Most randomised trials fail to represent the full spectrum of patients with an ethnic minority background treated in routine clinical practice. Patients unable to (adequately) speak the local language, required for informed consent, are generally not considered for randomised trials, resulting in enrolment of fewer ethnic minority patients; this also applies to the present study. Data regarding self-reported ethnicity of non-enrolled patients was not available, so it is not known how many ethnic minority patients did not participate. The possible language barrier could also lead to enrolment of younger patients in the ethnic minority group, as they are generally more proficient in the local language. Although we adjusted for age in the multivariate model, this could potentially still impact clinical outcomes. Health disparities between ethnic minority and Western European patients encompass not only cardiovascular risk profiles, but also social determinants of health [27, 28], which are difficult to measure and were not documented in this study. A particular challenge in studying ethnic disparities is the heterogeneity and genetic diversity within ethnic minority groups. Given the small size of most ethnic subgroups, we chose a straightforward approach that categorised trial participants as ethnic minority or Western European patients. Yet, we acknowledge that allocating all patients with an ethnic minority background to a single group combines ethnic subgroups with varying cardiovascular risk profiles and social health determinants. Nevertheless, we felt that the chosen pragmatic approach was sound and perhaps the only way to obtain meaningful results.