Discordance in Patient Classification Using T-Scores

Abstract

In their original study report, “Assessment of Fracture Risk and Its Application to Screening for Postmenopausal Osteoporosis,” the World Health Organization (WHO) explicitly stated that any T-score criterion for osteoporosis is sensitive to bone mineral density (BMD) measurement site and technique, as well as the young adult reference population. Yet, the T = −2.5 criterion introduced by WHO is used for many different BMD techniques, despite the fact that it was based primarily on the relationship between forearm measurements and prevalent hip fracture in postmenopausal Caucasian females. It is reasonable to expect that a T-score threshold of –2.5 may be inappropriate for different skeletal sites and measurement techniques. This may explain the large variation in osteoporosis prevalence observed when different skeletal sites are measured. In this study, we compared the prevalence of osteoporosis (based on the T = −2.5 criterion) at different skeletal sites using the manufacturer's normative data. We determined the expected mean T-score for a 60-yr-old Caucasian female at the heel (ultrasound), hip (dual X-ray absorptiometry [DXA]), spine (PA DXA, lateral DXA, and quantitative computed tomography [QCT]), and forearm (DXA). Assuming a normal distribution of T-scores at a fixed age, we computed the expected percentage of 60-yr-old Caucasian women that would be classified as osteoporotic using the –2.5 standard deviation criterion for each technique. At age 60 yr, the expected mean T-score ranged from –2.5 (spine QCT) to –0.7 (heel). Prevalence estimates ranged from 3% at the heel to 50% for spinal QCT. It was also noted that the sites with the strongest relationship to hip fracture risk (the hip and heel) showed the least age-related T-score decline and lowest estimated prevalence. We conclude that a single T-score criterion cannot be universally applied to all BMD measurements. The discrepancies in the prevalence of osteoporosis are the result of several factors, including differences in age-related bone loss at different skeletal sites, differences in the young adult reference populations used by the various bone densitometry devices, and technology-related differences. Using estimated BMD by heel ultrasound, few patients will have T-scores below –2.5, whereas most postmenopausal women will fall below this level for spine bone density measurements performed by lateral DXA or QCT. Based on these data, it may be necessary to provide a T-score criterion specific to the type of densitometric evaluation performed.