1. Research Centre for Metrological Traceability in Laboratory Medicine (CIRME) , Via GB Grassi 74 , 20157 Milan , Italy , Phone: +39 02 50319848, Fax: +39 02 39042896
2. Research Centre for Metrological Traceability in Laboratory Medicine (CIRME) , University of Milan , Milan , Italy
3. Reference Centre for External Quality Control, Careggi University-Hospital , Florence , Italy