“Study of vitamin D status of rheumatoid arthritis patients Rationale and design of a cross-sectional study by the osteoporosis and metabolic bone diseases study group of the Italian Society of Rheumatology (SIR)”, Reumatismo, vol. 58, no. 4, pp. 314–318, Dec. 2006, doi: 10.4081/reumatismo.2006.314.