BONE MINERAL DENSITY TESTING

The diagnosis of osteoporosis in women and men should be focused on determining the cause of low bone mineral density (BMD) and making a differential diagnosis of conditions associated with osteoporosis.

Obtaining a complete history is the initial step in the medical diagnosis of osteoporosis. Specific information the clinician should focus on include:

• Previous Fracture 
  Osteoporotic fractures occur at sites of low bone mass. These fractures may be non-traumatic, as usually seen in vertebral bodies, or induced by trauma such as falls, resulting in hip and wrist fractures. A patient's risk of suffering a vertebral compression fracture is five times higher if the patient has already had one vertebral compression fracture, and twelve times higher if there is a history of two prior vertebral fractures. The risk of hip fracture increases two fold if the patient has had a previous symptomatic vertebral fracture. A history of suffering a fracture after the age of 40 doubles the risk of sustaining another fracture.
  
• Genetics 
  A history of maternal hip fracture indicates an increased susceptibility to hip fracture independent of that attributable to low bone mass. Greater height, associated with a longer hip axis has also been indicated as an increase risk of hip fracture.
  
• Social Factors 
  The use of alcohol and/or tobacco and an active or sedentary lifestyle should also be determined, as research has indicated these are all risks associated with osteoporosis. Persons who exercise have a 10% to 20% increased bone density compared with those who are sedentary.
  
• Nutritional Status 
  Maintenance of optimal bone health depends on an adequate supply of calcium and other essential nutrients. Based on dietary data, a large percentage of Americans fail to meet currently recommended guidelines for optimal calcium intake. Adequate vitamin D is essential for optimal intestinal calcium absorption. Forty percent of men and 30% of women with hip fractures have been found to be vitamin D deficient. With advancing age, there is less efficient intestinal absorption of both vitamin D and calcium, and the kidney produces less active vitamin D.
  
• History of Previous Falls 
  Greater frailty, generalized weakness, poor balance, poor visual function and certain medications increase the risk of suffering an injurious fall by increasing the probability of a fall, and decreasing the patient's ability to use protective reflexes. Studies have shown that both the direction of a fall (to the side), and the absence of protective reflexes increases the risk of fracture independently of bone mass.

The physical portion of the medical examination should consist of objective measurements of height, posture, gait, gross range of motion, strength, balance and reflex testing. The estrogen deficient state of all postmenopausal patients should also be determined. Routine laboratory testing is performed to determine if medical conditions associated with bone loss are present. Appropriate testing should be pursued to determine a definitive diagnosis and to identify remedial causes of skeletal deterioration.

Though risk factors are important in helping to identify patients with possible osteoporosis, Bone Mineral Density (BMD) testing is the best way to confirm or rule out the diagnosis. X-ray studies of the thoracic and lumbar spines can help assess the extent of osteoporotic damage, but generally, x-ray films show bone loss only when it exceeds 30% or more. Another accepted technique is spine quantitative computed tomography. However, this technique has a relatively low precision, is complex to perform, and has a high cost. BMD measurement is performed using Dual-Energy X-ray absorptiometry (DEXA). It is recommended that patients suspected of osteoporosis be tested using the central hip and spine densiometer. These sites provide a good baseline and follow-up measurements if therapeutic intervention is planned.

The World Health Organization has established the following BMD based diagnostic criteria in using BMD results:

• Peak Bone Density - the mean bone density of a 30 year old control subject 
• Normal - a value for BMD greater than 1 standard deviation (SD) of the young adult mean 
• Osteopenia - a BMD value greater that 1 SD but less than 2.5 below the young adult mean value 
• Osteoporosis - a BMD value 2.5 SD or greater below the young adult mean

Most bone densiometry reports designate the standard deviations from the normal young adult mean as "T scores". Diagnostic criteria are usually stated as T scores because fracture risk is derived from epidemiological studies that use this designation as a reference. "Z scores" represent the standard deviations from age-and-sex-matched control subjects. This provides useful diagnostic information because a Z score of 2 or more below the age-and-sex-matched control may suggest a secondary cause of osteoporosis. For each 10% decrease in BMD, the fracture risk approximately doubles. The proportion of patients with a T score of 2.5 or less than the young adult mean increases exponentially with age, and the number of women with osteoporosis who currently have a T score in this range total approximately 6 - 7 million.

The Federal Bone Mass Measure Act, effective July 1, 1998, standardized Medicare reimbursement for BMD scanning and ensures access to BMD testing every two years, or more frequently if medically necessary, for qualified persons 65 years and older. Those qualified under Medicare include:

• estrogen deficient women at clinical risk for osteoporosis, 
• persons with vertebral abnormalities, 
• those receiving long-term steroid therapy,
• persons with primary hyperparathyroidism, and 
• those being monitored to assess an osteoporotic drug therapy.

The following guidelines have been suggested for physician's when factoring a patient's BMD results into the intervention decision:

• Preventive intervention when the T score is greater than 1.0 but less than 2.5 
• Therapeutic intervention when the T score is 2.5

Follow-up BMD measurements should be performed to monitor changes in bone mass. With the DEXA technique, a change of 5% is considered clinically significant, and is usually not observed in less than two years. Approximately 20% of women receiving treatment for osteoporosis may lose bone density during the first year of treatment, but with continued treatment, BMD gains in the second year usually exceed the losses of the first year.

Table 1. Suggested Guidelines for Follow-up BMD Testing