Evaluation of a fully automated serum assay for C-terminal cross-linking telopeptide of type I collagen in osteoporosis.

BACKGROUND: Biochemical markers of bone turnover can provide prognostic information about the risk of osteoporotic fracture and are useful tools for monitoring efficacy of antiresorptive therapy. A serum-based automated assay may be of better clinical value than urinary markers because of lower imprecision and day-to-day within-person variability. Our aim was to evaluate the technical and clinical performances of a new, fully automated assay for serum C-terminal cross-linking telopeptide of type I collagen (CTX), a marker of bone resorption.

METHODS: Serum CTX was measured on the Elecsys 2010 automated analyzer (Roche). Results were compared with those of the manual ELISA. We measured serum CTX concentrations in 728 healthy women, ages 31-89 years. We investigated the ability of this assay to predict the rate of postmenopausal forearm bone loss evaluated by four repeated bone mineral density measurements using dual-x-ray absorptiometry in 305 women followed prospectively for 4 years. Finally, in a cohort of healthy, untreated, postmenopausal women, we compared baseline serum CTX in 55 women who subsequently had a fracture (20 vertebral and 35 peripheral fractures) with values in the 380 women who did not fracture during a mean 5 years of follow-up.

RESULTS: The within- (n = 21) and between-run (n = 21) CVs were <4.1% and 5.7%, respectively. In 728 healthy women, serum CTX concentrations (automated) correlated with those of the manual ELISA (r = 0.82; P<0.0001). The median long-term within-person variability assessed by four repeated measurements over 3 months in 18 postmenopausal women was 9.4%. Compared with 254 premenopausal women, serum CTX was 39% (P<0.0001) higher in 45 perimenopausal women and 86% (P<0.0001) higher in 429 postmenopausal women (mean age, 64 years). Baseline serum CTX correlated negatively with changes of bone mass measured at the mid (r = -0.23; P<0.0001) and distal (r = -0.27; P<0001) radius. Postmenopausal women with serum CTX greater than the mean + 2 SD values in premenopausal women accounted for 42% of the population, lost bone at the mid radius on average eightfold more rapidly than the other women (-0.27% +/- 2.92% vs. -2.25% +/- 3.95%; P<0.0001), and had increased risk of fracture with a relative risk (95% confidence interval) of 1.8 (1.01-3.1) after adjustment for physical activity.

CONCLUSIONS: The automated assay for serum CTX is precise and predicts rate of bone loss and fracture risk in postmenopausal women. Because it is convenient to use and has high throughput, this serum bone resorption marker may be useful for the investigation of patients with osteoporosis.