Reference intervals for bone turnover markers in Spanish premenopausal women

Background: The aims of this study were to establish robust reference intervals and to investigate the factors influencing bone turnover markers (BTMs) in healthy premenopausal Spanish women. Methods: A total of 184 women (35-45 years) from 13 centers in Catalonia were analyzed. Blood and second void urine samples were collected between 8 a.m. and 10 a.m. after an overnight fast. Serum procollagen type I amino-terminal propeptide (PINP) and serum cross-linked C-terminal telopeptide of type I collagen (CTX-I) were measured by two automated assays (Roche and IDS), bone alkaline phosphatase (bone ALP) by ELISA, osteocalcin (OC) by IRMA and urinary NTX-I by ELISA. PTH and 25-hydroxy-vitamin D (25OHD) levels were measured. All participants completed a questionnaire on lifestyle factors. Results: Reference intervals were: PINP: 22.7-63.1 and 21.8-65.5 mu g/L, bone ALP: 6.0-13.6 mu g/L, OC: 8.0-23.0 mu g/L, CTX-I: 137-484 and 109-544 ng/L and NTX-I: 19.6-68.9 nM/mM. Oral contraceptive pills (OCPs) influenced PINP (p = 0.007), and low body mass index (BMI) was associated with higher BTMs except for bone ALP. Women under 40 had higher median values of most BTMs. CTX-I was influenced by calcium intake (p = 0.010) and PTH (p = 0.007). 25OHD levels did not influence BTMs. Concordance between the two automated assays for PINP and particularly CTX-I was poor. Conclusions: Robust reference intervals for BTMs in a Southern European country are provided. The effects of OCPs and BMI on their levels are significant, whilst serum 25OHD levels did not influence BTMs. Age, calcium intake, BMI and PTH influenced CTX-I. The two automated assays for measuring PINP and CTX-I are not interchangeable.