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Objective: This study aims to identify the prevalence of osteoporosis (OP) by lumbar computed tomography (CT) Hounsfield units (HUs) in patients who have normal or osteopenic bone determined by dual-energy x-ray absorptiometry (DEXA).Summary of Background Data: OP is a critical issue in the postmenopausal and aging population. Bone mineral density assessment by DEXA has been described as insensitive for diagnosing OP in the lumbar spine. Improving the detection of OP can bring more patients to treatment and reduce the risks associated with low bone mineral density. Patients and Methods:We retrospectively reviewed all patients with DEXA scans and noncontrast CTs of the lumbar spine over a 15-year period. Patients were diagnosed as non-OP if they had a normal DEXA T-score ( ≥ −1) or osteopenic DEXA T-score (between −1.1 and −2.4). Patients in this cohort were considered osteoporotic by CT if L1-HU ≤ 110. Demographics and lumbar HUs were compared between these stratified groups.Results: A total of 74 patients were included for analysis. All patients were demographically, similar, and the average patient age was 70 years. The prevalence of OP determined by CT L1-HU ≤ 110 was 46% (normal DEXA: 9%, osteopenic DEXA: 63%). A significant number of males in our study were considered osteoporotic by L1-HU ≤ 110 (74%, P = 0.03). All individual axial and sagittal lumbar HU measurements including L1-L5 average lumbar HUs were statistically significant among non-OP and OP groups except for the lower lumbar levels (P > 0.05 for L4 axial HUs, and L4-L5 sagittal HUs). Conclusions:The prevalence of OP in patients with normal or osteopenic T-scores is high. Among those with osteopenia by DEXA, more than 50% may lack appropriate medical treatment. The DEXA scan may be particularly insensitive to male bone quality making the CT HU the diagnostic method of choice for detecting OP.