Bone Health: Are Drugstore Bone Scans Accurate?
Q.
How reliable are the bone scans offered at the drugstore chains? I am hesitant to have a test that may be inaccurate and give a false-positive or false-negative reading.
A.
The accuracy of bone density testing depends on the method used. The "gold standard" is DEXA (dual-energy X-ray absorptiometry) of the hip and spine. This test is the one against which all other tests are measured, but it is expensive. Most of the portable machines in places such as drugstores use ultrasound of the heel. This is actually a fairly accurate method for predicting future risk of fracture, and it is relatively inexpensive. No matter the method used, bone mineral density is reported as the difference between the expected density for someone of the same age and gender (Z-score) and the average bone mineral density for a 35-year-old of the same sex (T-score). These scores are expressed as standard deviations from the mean. The T-score is the one used to predict osteoporosis and subsequent fracture risk -- after all, who wants to have the bone density of a 75-year-old!
A T-score above -1 (meaning that bone density is no worse than one standard deviation below that of a 35-year-old) is normal. A T-score between -1 and -2.5 (between 1 and 2.5 standard deviations below the density of a normal 35-year-old) indicates osteopenia, or bone thinning, while a score below -2.5 means osteoporosis. Osteoporosis increases the risk of fractures, especially of the hip, spine and wrist.
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