1. Hospital Universitario Fundacion Alcorcón; Spain
2. Nephrology Department; Hospital Universitario Gregorio Marañón; Spain
3. Hospital Universitario Gregorio Marañón; Spain
4. Complutense University; Medicine Faculty, Physiology Department; Spain
5. National Epidemiolgy Center; Instituto de Salud Carlos III; Spain
6. Nephrology Department; Hospital Universitario Doce Octubre; Spain