stored 1 week at -20 #{176}C and then shipped on solid CO2 to the Institut fur Standardisierung und Dokumentation im Medizinischen Laboratorium e.V. (INSTAND; D#{252}sseldorf, Germany); from there, some were shipped to the Instituut voor Hygiene en Epidemiologie (IHE; Brussels, Belgium). The samples were stored for 3 months at -20 #{176}C at both institutes before shipment to the participants. Tests for infectious diseases had been performed by Scantibodies and the sera were negative for HW-1/2 antibody, hepatitis C virus antibody, syphilis, and hepatitis B surface antigen. The target values were determined in the Laboratoria voor Medische Biochemie en Klinische Analyse (University of Ghent, Ghent, Belgium) by use of ID GC-MS [3-5]. The within-day CVs and the inaccuracy of all
