Comparison of clinical cut-points and treatment targets for urine NTX and plasma βCTX-I in osteoporosis

•Fracture outcome based treatment targets for bone turnover markers are lacking in the literature.•We identified plasma βCTX-I values comparable to currently used urine NTX treatment targets for osteoporosis.•We measured p-βCTX-I by 3 methods and uNTX in 146 patients and interpolated treatment targets from regression equations.•The p-βCTX-I equivalent to the fracture outcome based uNTX treatment-target of 21 nmol BCE/mmol, is approximately 250 ng/L.•This treatment target is lower than the mean of the reference interval for healthy premenopausal women.

ObjectiveWe undertook to identify levels for plasma β isomerised carboxy-terminal telopeptides of type I collagen (p-βCTX-I) that are comparable to currently used urine amino-terminal telopeptides of type I collagen (u-NTX) cut-points and treatment targets in osteoporosis.Design and methodsFasting morning samples were collected from patients attending tertiary hospitals and clinics for investigation of metabolic bone disease. Patients with Paget's disease or < 20 years of age were excluded. Second void spot urine for NTX and plasma (EDTA) samples were utilised. Urine was analysed routinely and plasma stored at − 20C until analysis by enzyme-linked immunosorbent assay (ELISA) (Immunodiagnostic Systems plc), E170 (Roche Diagnostics) and IDS-iSYS (Immunodiagnostic Systems plc) methods. The relationship of u-NTX with each p-βCTX-I method's results was assessed by Passing and Bablok regression, and p-βCTX-I levels equivalent to u-NTX cut-points and targets were interpolated.ResultsOne hundred and forty six patients were included. Spearman correlation coefficients ranged from 0.71 to 0.75 for the three βCTX-I assays. The equivalent βCTX-I concentrations for NTX/Cr values of 21 (fracture risk reduction target following risedronate therapy), 27 (healthy pre-menopausal women's mean value), and 38 (threshold for reduction of BMD on calcium alone) nmol BCE/mmol were 230, 312 and 462 ng/L for the automated Roche assay and 271, 395 and 624 ng/L for the automated IDS i-SYS assay respectively.ConclusionsThe p-βCTX-I equivalent to the only available fracture outcome based absolute treatment threshold of 21 nmol BCE/mmol established for u-NTX, is close to 250 ng/L but will vary between p-βCTX-I assays.