Bone Abstracts

Bone turnover markers provide an early, dynamic assessment of the skeletal response to osteoporosis treatment. The reference markers are serum procollagen type I N-terminal propeptide (PINP), which reflects bone formation, and serum C-terminal telopeptide of type I collagen (CTX), which reflects bone resorption. Their main clinical value is not in diagnosing osteoporosis, because most patients have values within the reference interval, but in monitoring treatment and supporting decisions when the response is uncertain. Pre-analytical and biological variability must be considered. CTX is particularly affected by circadian rhythm and food intake and should preferably be measured in a fasting morning sample. PINP is less variable and is often more convenient for routine practice. Recent fracture, renal impairment, other skeletal disorders and medications may also influence results. For oral bisphosphonate treatment, a baseline measurement followed by repeat testing after approximately 36 months can identify whether the expected antiresorptive response has occurred. A fall exceeding the least significant change indicates a biological response. In practice, a decrease in PINP of more than 10 µg/L, or suppression to below approximately 35 µg/L, provides a useful treatment target. Failure to meet either criterion should prompt a review of adherence, administration technique, absorption, secondary causes of high turnover, and the possible need for parenteral treatment. Zoledronate and denosumab produce rapid suppression of CTX followed by PINP. Teriparatide increases PINP, commonly by more than 10 µg/L, whereas romosozumab produces an early increase in formation markers accompanied by suppression of resorption. Markers may also help assess the persistence of bisphosphonate effects during a treatment pause. A subsequent rise in PINP beyond the least significant change or above the treatment target suggests waning antiresorptive activity. By contrast, denosumab should not be stopped without planned follow-on therapy because bone turnover rises rapidly and may overshoot pretreatment values. Used alongside clinical assessment and bone mineral density, bone turnover markers can improve confidence that treatment has been started correctly, is being taken, and is producing the expected skeletal response.