Although some epidemiological studies on the EGPA incidence have been reported in European countries, no such study has yet been performed in Poland. In the present study, we noted several interesting points concerning the EPGA epidemiology based on the inpatient discharge records. The estimation of the average annual incidence of EGPA in Poland amounted to 1.5 cases per million people. This finding is in line with those coming from other countries (Pamuk et al. 2016; Romero-Gomez et al. 2015; Sada et al. 2014; Vinit et al. 2011; Mohammad et al. 2009; Watts et al. 2008; Ormerod and Cook 2008). A population-based vasculitis register covering 2.78 million inhabitants of northern Germany over a period of 5 years has revealed a stable incidence of EGPA (Reinhold-Keller et al. 2005). Any fluctuations would be due to unknown factors and would not be related to environmental causes. The mean age of our patients with the first EGPA hospitalization was 49 years, which also was comparable to the 50.3 years in a study by Comarmond et al. (2013) or 55 years in that by Sada et al. (2014). We found that more patients lived in urban than rural areas, which could suggest the presence of risk factors associated with the environment or lifestyle. Such factors, however, could not be substantiated. By contrast, Herlyn et al. (2014) have found a significantly higher prevalence of EGPA in rural than urban regions in northern Germany.

The present study shows the male-to-female ratio of EGPA patients of 0.67. This ratio was 0.50 in a Japanese study (Sada et al. 2014). In another study, no gender predominance was noted, but female patients were significantly younger at the time of EGPA diagnosis compared with males (Comarmond et al. 2013). In yet another study performed in a large group of 2195 patients, a predominance of females has been found, with the participation of only 42.1% male patients (Hasegawa et al. 2015). In that study it has also been reported that in-hospital deaths occurred in 97 out of the 2195 patients (4.4%); a lower mortality rate was associated with female gender, and a higher rate concerned unscheduled admissions. In the present study, 10 deaths of EGPA patients (3%) took place. All deaths occurred in adults, six of whom were hospitalized with the first-time diagnosis of EGPA. A study by Phillip and Luqmani (2008) has reported a 68–100% survival rate for EGPA patients within 5 years. Comarmond et al. (2013) have reported a 5-year relapse-free survival rate of 58.1% (95% CI: 45.6–68.6) for ANCA-positive and 67.8% (95% CI: 59.8–74.5) for ANCA-negative patients; the difference between the two was statistically insignificant. That is in line with a report from a Polish tertiary referral center, which shows that the presence of ANCA does not appreciably affect the frequency of relapses, although ANCA status may cause differences in clinical presentation of EGPA (Sokolowska et al. 2014).

The present study has several limitations. The database used provided general epidemiological information regarding hospitalization of EGPA patients. Detailed risk factors and other correlates of the disease could not be assessed in individual patients. Another limitation has to do with relying on the hospital discharge records, which may have excluded outpatients. The appearance of EGPA diagnosis in the discharge records is not necessarily the date of the first diagnosis. This inaccuracy may lead to an overestimation of the count of incident cases. However, a long observation time in this study may have mitigated any possible overestimation. The disease is a systemic necrotizing vasculitis with multi-organ involvement which usually requires advanced diagnostic procedures that can be done only in inpatient settings.

The average annual incidence of EGPA in Poland was estimated at 1.5 per million people (95% CI: 1.2–1.8), and the point incidence was 8.8 per million at the end of 2013. These figures are comparable to those reported for other European countries. Hospital discharge records may be a useful tool for epidemiological studies on EGPA, a life-threatening rare systemic vasculitis.