The importance of laboratory medicine in the management of CKD-MBD: insights from the KDIGO 2023 controversies conference.

Laboratory investigations are important in the clinical management and the study of chronic kidney disease-mineral and bone disorder (CKD-MBD)- including CKD-associated osteoporosis. Parathyroid hormone (PTH) is the major hormone in the regulation of bone and calcium balance but is significantly affected in advanced CKD. Knowledge of PTH concentration is important in the assessment of osteoporosis including CKD-associated osteoporosis; however, measurement of PTH in the laboratory is bedevilled by interferences and inter-method differences compounded by lack of standardisation of commonly used immunoassays. Vitamin D is important for bone health and its deficiency contributes to the development of osteoporosis. Vitamin D metabolism is impaired in advanced CKD, augmenting the effects of its deficiency on bone health. Lack of consensus on optimal serum 25-hydroxyvitamin D (25-(OH)D) concentrations for bone health, including in the various CKD stages, is compounded by lack of analytical specificity of immunoassays. Liquid chromatography tandem mass spectrometry (LC-MS/MS) assays would help overcome these issues. Ionised calcium measurement is recommended for assessment of serum calcium, especially in CKD. Fibroblast growth factor 23 (FGF23) is important in the homeostasis of phosphate that accumulates in CKD, though this marker is not yet utilized in clinical context. Calculated maximal tubular reabsorption of phosphate normalized to glomerular filtration rate (TmP/GFR) may help in assessment of phosphate homeostasis. Bone specific alkaline phosphatase (BALP) and tartrate-resistant acid phosphatase isoform 5b (TRACP5b), the reference markers of bone formation and resorption for CKD-associated osteoporosis, have been shown to reflect bone turnover by histomorphometry in patients with advanced CKD.