Situation: A 68-year-old woman is seen at the outpatient clinic after a dual-energy X-ray absorptiometry (DXA) scan. She reached menopause at age 50, weighs 45 kg, smokes, and has taken oral prednisone for 1 year for a lung condition. Her serum calcium is 9.1 mg/dL (normal 8.6–10.2 mg/dL). Her T-scores are lumbar spine −2.2, total hip −1.8 and femoral neck −2.6. How should her result be classified?

In postmenopausal women, osteoporosis is defined by a T-score of −2.5 or below at the lumbar spine, total hip or femoral neck, and the classification uses the lowest of these sites. Her femoral neck value of −2.6 therefore means osteoporosis, even though her other sites fall in the low bone mass range (between −1.0 and −2.5).

Core diagnostic rule In postmenopausal women, the DXA-based diagnosis of osteoporosis is not an average of sites and is not decided by the majority of sites. The classification uses the lowest T-score among the lumbar spine, total hip, and femoral neck. A T-score of −2.5 or below at any one of these three sites establishes osteoporosis, while a T-score between −1.0 and −2.4 indicates low bone mass, or osteopenia ^[1][2].

Applying the rule to this patient The patient’s values are lumbar spine −2.2, total hip −1.8, and femoral neck −2.6. The femoral neck is the lowest value and crosses the −2.5 threshold, so the result is classified as osteoporosis. The fact that the other two sites remain above −2.5 does not change the diagnosis, because a single osteoporotic site is sufficient to confer the diagnosis and the associated fracture risk ^[1].

Why the lowest site matters clinically Bone strength and fracture risk vary by skeletal site, and the femoral neck is especially important because hip fractures carry the greatest morbidity, mortality, and quality-of-life burden in older adults ^[2]. A femoral neck T-score of −2.6 therefore signals a clinically meaningful reduction in bone strength at a high-consequence site, even when the lumbar spine and total hip appear less affected. The diagnostic framework is intentionally conservative: using the lowest site prevents underestimating fracture risk when one region is disproportionately weakened ^[1][3].

Measurement variability and the −2.5 cutpoint DXA T-scores are not perfectly precise. The 95% confidence interval around the femoral neck T-score is approximately ±0.41, meaning that small differences around the −2.5 threshold may not be statistically significant ^[3]. In this case, however, the femoral neck value of −2.6 falls within the range where the diagnosis of osteoporosis remains appropriate by the standard T-score criterion. When a value sits very close to −2.5, repeat measurement or consideration of clinical risk factors may be warranted, but the classification rule itself does not change ^[3].

Secondary causes and the patient’s risk profile The patient’s history includes several factors that can reduce bone mass: early menopause at age 50, low body weight of 45 kg, smoking, and 1 year of oral prednisone. Glucocorticoid use is a well-recognized secondary cause of bone loss, and in postmenopausal women with a T-score of −2.5 or below, secondary contributors such as osteomalacia or medication effects should be considered rather than assuming estrogen deficiency alone . Her serum calcium of 9.1 mg/dL is within the normal range, but a normal calcium level does not exclude osteoporosis or a mineralization defect, and it does not alter the T-score-based classification .

Key point! Osteoporosis is diagnosed by the lowest T-score at the lumbar spine, total hip, or femoral neck. A single site at −2.5 or below is enough, even if other sites are only osteopenic.

Watch out! Do not average the three T-scores or require two abnormal sites. The femoral neck value of −2.6 alone makes this osteoporosis, not low bone mass.