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.

Background: Chronic kidney disease (CKD) is characterized by persistent inflammation, metabolic dysregulation, and mineral and bone disorder (CKD-MBD), all of which may affect ocular tissues. While retinal manifestations of CKD have been increasingly investigated, data regarding clinical anterior segment modifications and their relationship with systemic biochemical parameters remain limited. Methods: We performed a cross-sectional study including 130 participants: 53 non-CKD subjects, 29 patients undergoing maintenance hemodialysis (HD), and 48 kidney transplant recipients (KTRs). All participants underwent comprehensive ophthalmic examination, including best-corrected visual acuity (BCVA), intraocular pressure (IOP) measurement using Goldmann applanation tonometry, Schirmer I test, tear break-up time (TBUT) and assessment of cataract and blepharitis prevalence. Serum creatinine estimated glomerular filtration rate (eGFR), calcium, phosphate, parathyroid hormone (PTH), magnesium, 25-hydroxyvitamin D, and Activin A were recorded. Associations between ocular and systemic parameters were evaluated using correlation and regression analyses. Results: TBUT, Schirmer I and IOP did not differ significantly among groups (all p > 0.05). After adjustment for age, sex, BMI and diabetes mellitus (DM), HD patients showed worse visual acuity than the non-CKD reference group, reflected by higher logMAR BCVA values (adjusted B = +0.128, p = 0.024). In KTRs, higher corrected calcium was independently associated with reduced tear film parameters, correlating with both shorter TBUT (B = −0.903, p = 0.037) and lower Schirmer I values (B = −5.947, p = 0.008). Circulating Activin A increased progressively from controls to KTR to HD patients (median 189.5 vs. 273.0 vs. 314.0 pg/mL, p < 0.001) but showed no independent association with any anterior segment parameter; it was instead associated with cumulative glucocorticoid exposure (B = +1.602, p = 0.024). Conclusions: In CKD, clinical anterior segment and ocular surface differences were largely explained by age, sex, and comorbidity rather than renal replacement modality, with visual acuity deficit persisting in HD after adjusting for these covariates. At the individual level, corrected calcium was the parameter most consistently associated with tear film parameters in KTRs, whereas elevated Activin A was not independently associated with ocular involvement and likely reflects systemic disease activity.