Statistics for European Biological Variation Study (EuBIVAS): within- and between-subject biological variation estimates of beta-isomerized C-terminal telopeptide of type I collagen (beta-CTX), N-terminal propeptide of type I collagen (PINP), osteocalcin, intact fibroblast growth factor 23 and uncarboxylated-unphosphorylated matrix-Gla protein-a cooperation between the EFLM Working Group on Biological Variation and the International Osteoporosis Foundation-International Federation of Clinical Chemistry Committee on Bone Metabolism