This is an open label, time series trial. The trial is likely to be single centre (additional sites will only be opened if necessary) and will involve 25 children with chronic kidney disease (stage 3b, 4-5) or on dialysis. The overall aim of this trial is to explore the viability of the Ca isotope ratio measured by dual-tracer stable Ca isotope method as a measure of bone mineral (Ca) content, and to evaluate how it changes in response to two commonly used medications that either contain Ca (calcium carbonate) or do not (sevelamer carbonate). Both calcium carbonate and sevelamer carbonate are routinely used in children, but their effect on the bone mineral content (measured by the Ca isotope ratio) has not been studied. This short-term trial will provide proof-of-concept data to determine the utility of the Ca isotope fractionation technique in guiding medication usage in children with CKD and on dialysis. These data will inform a potential future randomised trial that utilises the calcium isotope fractionation technique to adjust the calcium intake (through diet and different medications, including vitamin D analogues) and monitor changes in important patient level outcomes such as fractures and bone mineral density on DXA scan. Participants will be administered sevelamer carbonate first for 16 weeks and then will switch to calcium carbonate for 12 weeks. Participants may need to change medication earlier than 16 weeks at the clinician's discretion based on their calcium levels on routine blood tests. Calcium isotope levels will be measured in blood and urine samples for up to 28 weeks. Isotopes levels in faeces and dialysis fluid samples may also be measured. This is not a Clinical Trial of an Investigational Medicinal Product (CTIMP).
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Ca isotope ratios in serum measured by dual-tracer stable isotope method (Ca isotope fractions Γ44/42Ca) of a child after up to 12 weeks treatment with each of two P-binders (Ca based and Ca free).
Timeframe: Measured over 12 weeks. N.B. Treatment changes from sevelamer carbonate to calcium carbonate after 16 weeks in the trial or earlier at the clinician's discretion based on serial monitoring of serum Ca levels.