1 Central Institute for Clinical Chemistry and Laboratory Medicine, Klinikum Stuttgart, Kriegsbergstrasse 60, D-70174, Stuttgart, Germany. Electronic address: m.shipkova@klinikum-stuttgart.de.
2 Department of Clinical Laboratory and Clinical Pharmacology, Alexander University Hospital, Faculty of Medicine, Medical University of Sofia, St. G. Sofiiski 1 Blvd., 1431 Sofia, Bulgaria. Electronic address: dsvinarov@yahoo.com.