CYP27A1 and osteoarthritis: The guidelines published by the European Society for Clinical and Economic Aspects of Osteoporosis and Osteoarthritis (ESCEO) and the International Osteoporosis Foundation (IOF) recommend collecting serum calcium and phosphorus levels, intact parathyroid hormone (iPTH), 25-OH- Vitamin D and at least one resorption (i.e. the C-terminal telopeptide, CTX) and one formation bone biomarker (i.e. N-terminal propeptide of type-I procollagen, P1NP or bone alkaline phosphatase) [91].