Damasiewicz MJ, et al. - Authors evaluated the variability of fibroblast growth factor-23 (FGF-23) levels in a cohort of stable haemodialysis patients. They also assessed the effects of different collection methods on measured FGF-23 levels in a subset of patients. Compared to plasma, the measured FGF-23 levels were significantly lower in serum as compared to plasma, and the addition of a protease inhibitor did not confer an additional benefit. Importantly, marked intra- and inter-individual variability was shown by FGF-23 levels in this cohort of end-stage kidney disease (ESKD) patients. In ESKD, the routine measurement of FGF-23 remains challenging, nonetheless, this study demonstrated the plasma to be the optimal collection method for FGF-23 analysis.