Reference intervals of bone turnover markers in healthy premenopausal women: Results from a cross-sectional European study

Robust validated reference intervals for bone turnover markers (BTMs) are required to assess fracture risk and effectiveness of therapy. However, there are currently limited reference intervals for BTMs in premenopausal women, especially comparing manual and automated assays. This study determined the BTM reference intervals using automated and manual assays, compared the results obtained from two different assays, and evaluated the factors that may affect BTM levels. This was a cross-sectional registry study in 194 healthy, premenopausal, European Caucasian women aged 35 to 39 years from France (n = 98) and Denmark (n = 96). Two independent specialized laboratories, one in France (Synarc) and the other in Denmark (Nordic Bioscience), analyzed blood and urine samples from each woman for BTM levels.The type of assay used in this study significantly affected the reference intervals obtained for serum cross-linked C-terminal telopeptide of type I collagen (sCTX) and urinary cross-linked N-terminal telopeptides of type I collagen (uNTX/Cr; both P < 0.001), but not for serum procollagen type I amino-terminal propeptide (PINP; P = 0.28). The Serum Crosslaps® ELISA; Microtitre-plate based ELISA; Metra BAP EIA; and UniQ® PINP RIA assays yielded higher BTM reference values. The reference intervals for the BTMs, as measured with Serum β-Crosslaps, Elecsys® 2010 Systems; VITROS® ECI System; Ostase®, Access® Immunoassay System; and Total PINP, Elecsys® 2010 Systems assays, were 0.111–0.791 ng/mL for sCTX, 12.3–59.7 nmol BCE/mmol creatinine for uNTX/Cr, 5.8–17.5 ng/mL for bone alkaline phosphatase (ALP), and 17.3–83.4 ng/mL for PINP, respectively.When measured with Serum Crosslaps® ELISA, Microtitre-plate based ELISA, Metra BAP EIA, and UniQ® PINP RIA, the reference intervals were 0.177–0.862 ng/mL for sCTX, 22.6–95.7 nmol BCE/mmol creatinine for uNTX/Cr, 14.8–38.8 U/L for bone ALP, and 19.5–75.2 ng/mL for PINP, respectively. The clinical interpretation of the BTMs of a subject should be based on comparison with a BTM reference interval, measured with the same assay method.

► Assay type significantly affects the reference intervals for sCTX and uNTX/Cr, but not for PINP.
► The reference interval for the same analyte differs by assay.
► Smoking, serum creatinine, and BMI affect uNTX/Cr and serum bone ALP levels, but age did not.
► These factors do not affect sCTX or PINP.
► BMI, smoking, 25-hydroxy vitamin D levels, and country affect parathyroid hormone level.