Bone mineral density and trabecular bone score in adults with primary immunodeficiencies: a cross-sectional study.

Introduction: Osteoporosis is increasingly recognized as an immune-mediated condition ("immunoporosis"), yet data on bone health in adults with primary immunodeficiencies (PIDs) remain limited. Chronic immune dysregulation and multisystem complications in PIDs may adversely affect skeletal integrity, but bone microarchitecture and muscle mass have not been comprehensively evaluated in this population. The aim of our study was to assess bone mineral density (BMD), trabecular bone score (TBS), and muscle mass in adults with PIDs. Exploratory analyses examined associations between these measures and clinical features and the main peripheral blood lymphocyte subsets. Methods: In this single-center cross-sectional study, 61 adult patients with PIDs underwent dual-energy X-ray absorptiometry (DXA) of the lumbar spine, total hip, femoral neck, and whole body. TBS was derived from lumbar spine DXA images, and the appendicular lean mass index was calculated to assess muscle mass. Clinical characteristics and immunological parameters, including peripheral blood lymphocyte subsets, were analyzed in relation to skeletal outcomes. Results: Based on age-appropriate DXA findings and clinical data, osteoporosis was diagnosed in 12 of 61 patients (19.7%). Three additional patients were diagnosed based on fragility fractures, increasing the overall prevalence to 15 of 61 (24.6%). Low BMD was more frequently detected at the hip than at the lumbar spine. Partially degraded or degraded TBS was observed in 15 of 61 patients (24.6%). Low muscle mass was identified in 17 of 61 patients (27.9%) and was associated with significantly lower BMD at all skeletal sites. Participants with bronchiectasis had lower femoral neck and total hip BMD and lower TBS, whereas those with enteropathy had lower BMD across all skeletal sites and lower TBS. Among participants eligible for T-score assessment (n = 15), NK-cell counts correlated positively with femoral neck (ρ = 0.63, p = 0.01) and total hip T-scores (ρ = 0.68, p < 0.01), and with TBS in the entire cohort (n = 61; ρ = 0.28, p = 0.03). CD4+, CD8+, and CD19+ cell counts were not associated with any evaluated densitometric parameters. Discussion and conclusions: In this cohort of adults with PIDs, reduced BMD and impaired TBS represented distinct, partially overlapping findings. Hip DXA appeared particularly informative, while TBS provided complementary information beyond BMD. These exploratory findings should be confirmed in prospective controlled studies.