Critical differences in the serial measurement of three biochemical markers of bone turnover in the sera of pre- and postmenopausal women

Objectives: The purpose of this investigation was to quantify the biologic, day-to-day variability and critical differences in serum levels of crosslinked collagen N-telopeptides (NTx), procollagen aminoterminal extension propeptides (PINP) and bone specific alkaline phosphatase (bAP) in healthy women. Design and methods: Seven blood samples were collected from12 pre- and 15 postmenopausal women over 4 to 6 months. NTx, PINP and bAP levels were determined utilizing enzyme- and radioimmunoassay techniques. Results: The within-subject coefficient of variation (C.V.) in serum bAP, NTx and PINP levels was 7.1, 10.6 and 12.4% respectively. These variances did not differ significantly among premenopausal women when compared with postmenopausal subjects. Combining terms for analytical and biologic variability revealed that a critical difference between 2 successive serial measurements is 24% for bAP, 34% for NTx and 38% for PINP. Conclusion: Circulating levels of NTx, PINP and bAP are stable over time periods of several months, allowing for the determination of significant changes in skeletal metabolism of women.