Components of biological variation of biochemical markers of bone turnoverin Paget's bone disease

The aims of this study were to evaluate the components of biological variat ion of the new markers of bone turnover in patients with Paget's bone disea se and to compare the results,vith data obtained in healthy subjects, Fifte en patients with Paget's disease in a stable period of the disease and 12 h ealthy premenopausal women were included for a 1 year follow-up study. With in- and between-subject biological variation, indices of individuality, and critical differences were evaluated for the following biochemical markers: in serum, total (tAP), and hone (bAP) alkaline phosphatases, procollagen t ype I N-terminal propeptide (PINP) and beta-carboxyterminal telopeptide of type I collagen (sCTx); in urine, hydroxyproline (Hyp), and amino (NTx) and beta-carboxyterminal (CTx) telopeptides of collagen type I. Serum markers of bone turnover showed lower biological variability than urinary markers. Within-subject biological variation was higher in pagetic patients than in healthy subjects for all serum markers, In both groups, bAP presented the l owest within-subject biological variation. In pagetic patients, all markers presented indices of individuality of < 0.6, indicating their usefulness F or patient monitoring. Critical differences were lower for serum markers th an for urinary markers. Among pagetic patients, serum bAP and PINP showed t he lowest critical differences with values close to 30%, whereas urinary CT x presented the highest critical differences (near 70%). Conversely, in hea lthy subjects, tAP was the marker with the lowest critical differences, bei ng two-fold higher in pagetic patients. This study confirms the lower sensi tivity of urinary markers to detect significant changes and indicates that data obtained on biological variations from healthy populations cannot alwa ys be extrapolated to pathological conditions. In addition, serum bAP and P INP seem to be the markers that best reflect a significant change in activi ty of Paget's disease, (Bone 26:571-576; 2000) (C) 2000 by Elsevier Science Inc. All rights reserved.