Bone Turnover Markers, DXA and Mineral Metabolism in Long-Term Kidney Transplant Recipients: A Cross-Sectional Comparative Study

Background: Bone disease remains a major long-term complication after kidney transplantation. However, the relationship between circulating bone turnover markers (BTMs) and densitometric skeletal parameters in kidney transplant recipients (KTRs) remains incompletely understood. Methods: This single-center, cross-sectional comparative study included 50 stable adult KTRs and 56 non-transplanted individuals with dual-energy X-ray absorptiometry (DXA)-confirmed osteopenia or osteoporosis. Serum bone formation markers, bone-specific alkaline phosphatase (BALP) and procollagen type I N-terminal propeptide (P1NP), and bone resorption markers, β-C-terminal telopeptide of type I collagen (β-CTX) and tartrate-resistant acid phosphatase 5b (TRAP5b), were quantified by enzyme-linked immunosorbent assay (ELISA). Bone mineral density (BMD), Z-scores, and Fracture Risk Assessment Tool (FRAX) estimates were obtained by DXA. Between-group comparisons were adjusted for age, sex, body mass index (BMI) and postmenopausal status, with sensitivity analyses restricted to participants with low bone mass. An exploratory receiver operating characteristic (ROC) analysis was performed to assess the association of BTMs with KTR status. Results: KTRs showed lower DXA-derived Z-scores at the total hip, femoral neck and lumbar spine despite being younger. After adjustment for age, sex, BMI and postmenopausal status, KTRs had significantly higher BALP (adjusted Δ = 129.96 ng/mL, p < 0.001) and PTH, and lower eGFR and magnesium than the reference group, whereas unadjusted differences in P1NP and lumbar spine BMD did not persist; β-CTX and TRAP5b did not differ between groups. BALP remained significantly higher in KTRs across the low-bone-mass restricted and propensity-score overlap analyses. BTMs showed no significant correlations with DXA parameters in KTRs. In the exploratory ROC analysis of study-group status, BALP showed the strongest separation between KTRs and the reference group (AUC = 0.808, p < 0.001). Conclusions: Long-term KTRs exhibit a distinct bone remodeling profile characterized most consistently by elevated BALP, which remained significant after adjustment and showed the strongest separation regarding KTR status. The limited BTM–DXA associations require confirmation in longitudinal studies.