Denosumab is a common and convenient treatment option for patients with osteoporosis. The cessation of denosumab can be associated with a transient but significant increase in bone turnover associated with bone loss and it is recommended that patients should be administered alternative anti-resorptive agents to mitigate this risk. 1,2 One approach suggests that CTX could be used to guide timing of administration of bisphosphonates such as zoledronic acid. 3 The cut-off for commencement of zoledronic acid ranges from 'above the mean found in age- and sex-matched cohorts', 3 to twice the upper limit of normal in premenopausal women 4 to absolute cut-offs. We measure CTX on the Roche Cobas analyzer in our laboratory. Reference intervals for CTX change depending on age and gender. We reviewed the data of over 200,000 CTX values measured from 2017 to 2023 to derive age specific percentiles in our local population.