BONE-DENSITY CHANGE AND BIOCHEMICAL INDEXES OF SKELETAL TURNOVER

Although biochemical markers of skeletal turnover cannot replace bone density scanning for the diagnosis of osteoporosis, it is thought that they may help add to prediction of fracture risk and help determine a dequacy of osteoporosis therapy. Nevertheless, whether biochemical mar kers in the serum or urine can predict individual rates of bone loss i n the spine or hip region is unknown. We studied a heterogeneous group of women (n = 81) who were pre menopausal, untreated postmenopausal, and estrogen-mass treated postmenopausal with baseline determination o f body mass index (BMI), calcium intake, biochemical measurements, and serial bone densitometry over 3 years. Serum assays included bone Gla protein (BGP), total and bone-specific alkaline phosphatase (AP, BSAP ): carboxyterminal propeptide of type I procollagen (PICP), carboxyter minal telopeptide of type I collagen (ICTP) and tartrate-resistant aci d phosphatase (TRAP). Urine assays included hydroxyproline (OHP), calc ium, total pyridinoline, and total deoxypyridinoline. Individual bioch emical markers and calcium intake were modestly correlated with bone d ensity changes but were inconsistent regarding the spine versus the hi p. All of the formation variables were significantly correlated to spi ne density change (r = -0.24 to -0.49) whereas the only resorption var iable that correlated was urine OHp/Cr (r = -0.31). The only formation variable that correlated with hip density change was serum PICP where as all of the resorption variables except serum TRAP were correlated ( r = -0.23 to -0.35). ''High turnover individuals were defined as those with levels of biochemical variables at least 1 SD above the mean you ng normal for each variable. Higher bone loss rates were seen in this group for several of the turnover markers compared with bone loss rate s in all other individuals. However, the sensitivity of this ''high tu rnover'' status for identifying high bone losers did not exceed 60% fo r any of the variables. In untreated postmenopausal women, a model usi ng urine OHp, serum ICTP, serum BSAP, and calcium intake was able to p redict 42% of the variance of change in BMD of the lumbar spine. A mod el using BMI, serum ICTP, and serum BGP could predict 32% of the varia nce of change in BMD of the femoral neck. No combination of markers co uld predict variance in bone density change at either site in estrogen ized women (premenopausal and estrogen-treated postmenopausal). We con clude that measuring individual serum and urine markers of bone turnov er cannot accurately predict bone loss rates in the spine and hip; how ever, combinations of demographic and biochemical variables could pred ict some of the variance in untreated postmenopausal women. Biochemica l markers cannot replace serial bone densitometry for accurate determi nation of change in bone mass at the most clinically relevant sites.