1. Centre for Clinical Research, Hospital of Västmanland Västerås, SE-721 89 Västerås, Sweden
2. School of Medical Sciences, Faculty of Medicine and Health, Örebro University, SE-701 82 Örebro, Sweden
3. Department of Laboratory Medicine, Faculty of Medicine and Health, Örebro University, SE-701 82 Örebro, Sweden
4. Statens Serum Institut, Artillerivej 5, 2300 Copenhagen S, Denmark