CT-Derived Hounsfield Units vs Dual-Energy X-Ray Absorptiometry in Spine Surgical Candidates: Concordance, Discordance, and Clinically Actionable Thresholds

Study Design Retrospective study. Objective To evaluate diagnostic concordance between CT–derived Hounsfield Units (HU) and DXA T-scores in spinal surgery candidates, and to identify factors related to discordance. Methods We analyzed 180 patients (mean age 72.4 ± 8.2 years) who had lumbar CT and DXA. DXA osteoporosis was defined by the lowest T-score (lumbar spine or hip) ≤ −2.5, and CT osteoporosis was defined as HU ≤ 100. Patients were classified into concordant positive (DXA+/HU+), concordant negative (DXA−/HU−), and two discordant groups (DXA+/HU−, DXA−/HU+). Analyses included correlation, ROC analysis, and comparisons of age, BMI, sex, and the DXA site yielding the lowest T-score. Results HU and T-scores showed correlation (Spearman’s ρ = 0.467, P < 0.001) with discrimination (AUC = 0.700; 95% CI 0.614-0.781). Concordance was 72.2% (130/180; DXA−/HU− = 94, DXA+/HU+ = 36); discordance was 27.8% (50/180; DXA+/HU− = 24; DXA−/HU+ = 26). DXA+/HU + patients were older than DXA−/HU− (75.9 ± 6.5 vs 71.1 ± 9.1 years; P = 0.003), and both DXA + groups had lower BMI (22.9 ± 3.8 and 22.7 ± 4.8 vs 24.8 ± 3.8 kg/m 2 ; P = 0.009 and 0.029). The HU 100-150 “gray zone” was not associated with discordance (23.3% vs 30.8%, P = 0.311). Conclusions HU values show moderate agreement with DXA and are useful for opportunistic screening. Given ∼27% discordance—especially in older or lower-BMI patients—HU should be interpreted alongside DXA for comprehensive assessment.