Longitudinal evaluation of a bone resorption marker in elderly subjects

The variability over time in the excretion of a bone resorption metabolite (collagen type I N-telopeptide crosslink, NTx) was evaluated in a cohort of community-dwelling elderly men and women (mean age 73 years). Three annual 24-h urine samples were collected. NTx concentration was measured using an established ELISA. Total (24-h) NTx excretion as well as Ntx/creatinine concentration were compared. Men had a significantly lower excretion of NTx/creatinine than women who were not on hormone replacement therapy. Overall, the within-subject long-term coefficient of variability for Ntx/creatinine was 26%. The correlation coefficient between the samples taken a year apart was higher for the 24-h NTx excretion (r = 0.66) than for the 24-h creatinine excretion (r = 0.51). The consistency of NTx excretion over time was also evaluated in all 93 subjects with three yearly samples using Kendall's rank correlation method; the resulting coefficient of concordance was 0.78 (significant at the 0.01 level). These results indicate that while NTx excretion varies in subject samples collected over a period of 2 years, this variability is not much greater than the daily variation reported for NTx and other bone metabolism markers. The relative reproducibility of NTx excretion over time in this age group was also evident in the coefficient of concordance. The results provide support for stratifying subjects according to level of bone resorption and identifying those subjects with high turnover who may be at greater risk of osteoporotic fracture.