Predicting Skin-to-Bone Depth During Bone Marrow Biopsy: The Role of BMI, Waist Circumference, and Sex

Background Bone marrow biopsy is an essential diagnostic procedure in haematology. Increasing rates of obesity may create technical challenges during biopsy by obscuring anatomical landmarks and increasing the distance between the skin surface and the posterior superior iliac spine (PSIS). Identifying patients who may be unsuitable for a standard non-image-guided approach could improve procedural planning, safety, and efficiency. Methodology Data from 50 consecutive patients undergoing bone marrow biopsy were collected as part of routine clinical care and a service evaluation. A retrospective analysis of these data was subsequently performed, with 46 patients included in the final analysis. Body mass index (BMI), waist circumference, and sex were recorded for each patient. Skin-to-bone depth at the PSIS was measured intra-procedurally. Associations were analysed using correlation analysis, simple linear regression, and multiple linear regression. Internal validation of the final regression model was performed using 1,000 bootstrap resamples to assess model stability and estimate optimism. Results Waist circumference and BMI were both significantly positively associated with skin-to-bone depth (p < 0.001 for both variables), with waist circumference demonstrating a stronger association (r = 0.617, R² = 0.381) than BMI (r = 0.489, R² = 0.239). Female patients demonstrated a greater mean skin-to-bone depth than males (4.74 cm vs 2.33 cm). A multiple linear regression model incorporating waist circumference and sex explained 61.1% of the variance in skin-to-bone depth (R² = 0.611). Internal validation using 1,000 bootstrap resamples demonstrated relatively stable model performance, with an optimism-corrected R² of 0.569. Although a waist circumference-by-sex interaction term was explored, it was not statistically significant. Conclusions Although waist circumference and sex were significant predictors of skin-to-bone depth, the predictive accuracy of anthropometric measurements alone was insufficient for reliable clinical decision-making. Substantial unexplained variability remained despite improved predictive performance in the combined regression model. Anthropometric measurements alone were therefore insufficient to reliably identify patients unsuitable for a standard non-image-guided biopsy. Clinical assessment remains essential when planning bone marrow biopsy procedures.