1. Department of Rheumatology, Clinic of Rheumatology, University Hospital St. Ivan Rilski, Medical University of Sofia, 13 Urvich St., 1612 Sofia, Bulgaria
2. Medical Faculty, Sofia University, St. Kliment Ohridski, 1 Kozyak Str., 1407 Sofia, Bulgaria
3. Faculty of Medicine, Medical University of Sofia, 1 Georgi Sofiiski Str., 1431 Sofia, Bulgaria