Author: Simon Kipersztok, M.D., University of Florida

Diagnosis of Osteoporosis

The presence of osteoporosis can be suspected from an assessment of risk factors (see Table 2). Yet, evaluation of risk factors alone for the determination of risk of fracture can fail to identify a sizable number of individuals who are at risk of fracture. Only BMD measurements can accurately assess the risk for fracture. BMD is the equivalent of blood pressure measurements for the prevention of stroke and assessment of lipid profiles for the prevention of cardiovascular disease. As BMD decreases the incidence of fractures increases in an exponential fashion [4].

There are a number of techniques available for BMD measurement and they all can predict risk of fracture (Figure). Dual X-Ray absorptiometry (DEXA) is the most widely used because of its excellent precision and accuracy, low radiation exposure, reasonable cost and length of time needed to scan a patient. The most common anatomical sites for assessment of risk of fracture are the lumbar spine and the non-dominant proximal femur. Measurement of BMD at any anatomical site can predict the risk of fracture at a distant site. However, the measurement of a given site is most accurate for its own risk of fracture.

Absorptiometry studies usually report the patient's BMD and two additional calculations at each measured site. One of the calculations compares the patient's BMD to the BMD measured at the same anatomical site in a population of individuals who are at an age when peak bone mass is attained. This is commonly referred to as the "Z score" or the "% young adult". This value predicts the patient's risk of fracture. The second calculation compares the patient's BMD to the BMD measured at the same anatomical site in a population of individuals who have the same age as the patient. This is commonly referred to as the "T-score" or the "% aged matched". This value indicates if factors other than age can be affecting the patient's BMD. Both values are expressed as standard deviations (SD) above or below the mean and are controlled for weight, ethnic origin and gender (see Figure 3).

The National Osteoporosis Foundation and the World Health Organization have established criteria for identifying patients at risk of fracture based on their BMD studies. For each 1 SD below the mean of the Z score the risk of fracture approximately doubles. Z scores within 1 SD of the mean are considered normal while Z scores that are between 1 SD and 2 to 2.5 SD below the mean are considered to be consistent with osteopenia. Z scores more than 2 to 2.5 SD below the mean are considered to indicate established osteoporosis.

Recently, biochemical markers of bone turnover have become commercially available and some markers can be measured in the serum while others in urine. The markers, unlike BMD measurements, are able to detect acute metabolic changes in bone turnover. While BMD can detect bone density changes in months and years, the bone markers can show changes in days and weeks. There are few clinical circumstances, other than in the process of evaluating and testing new drugs, where the clinician needs to know within days or weeks the metabolic status in the bones of patients undergoing prevention of or therapy for osteoporosis. Therefore, the practical clinical utility of bone markers to date is limited. An additional limitation is the fact that there are markers of bone formation and markers of bone resorption, each having its own biological variation and its own individual requirements in terms of sample collection and laboratory measurement which makes the decision of which marker to use somewhat cumbersome. One possible clinical utility of this technology is in the assessment of compliance to therapy. To date there are no guidelines available for the use of bone turnover markers [5].