Plain radiography is of limited value. Osteoporosis is not detectable until 40–50% of bone is lost.
25-hydroxy vitamin D (most useful test): normal range 75–250 nmol/L.
Plasma calcium, phosphate and alkaline phosphatase, parathyroid hormone (usually normal).
Thyroid stimulating hormone.
Consider tests for multiple myeloma in an osteoporotic area.
Densitometry can predict an increased risk of osteoporosis and fracture, the best current modality being dual energy X-ray absorptiometry (DEXA scan) in a facility with high-standard quality control.3 The spine and femoral neck are targeted: the femoral neck is the most useful index.
DEXA, T scores and Z scores5
DEXA is the current gold standard for the diagnosis of osteoporosis. It assesses both whole-body and regional bone mass (lumbar spine and proximal femur). Bone mass is measured as bone mineral density (BMD) in g/cm2 and the lower the BMD, the higher the risk of fracture. There are actually different normal ranges of BMD for each bone and for each type of DEXA measuring machine.
The BMD ‘T score’ is the number of standard deviations (SD) away from the mean BMD of a 30-year-old adult (see TABLE 89.2). Osteopenia (low bone density) is 1–2.5 SDs below the young adult standard mean. Osteoporosis is >2.5 SD below this mean. This is a strong indicator of bone fragility. Consider treatment if T score is <−2.5.
Table 89.2Interpretation of T scores (WHO criteria) |Favorite Table|Download (.pdf) Table 89.2 Interpretation of T scores (WHO criteria)
|T score ||Interpretation |
|≥−1.0 ||Normal |
|−1 to −2.5 ||Osteopenia |
|≤−2.5 ||Osteoporosis |
|<−2.5 with fracture ||Severe osteoporosis |
The BMD ‘Z score’ is the number of SDs away from the age- and sex-matched mean BMD. The Z score is used to express bone density in patients <50 years, premenopausal women, younger men and children. If low (<−2) it indicates prompt investigation for underlying causes of a bone deficit.
BMD is subsidised under the MBS6 for:
people over the age of 70 years (2- or 5-yearly screening, depending on first T score)
diagnosis of osteoporosis following a low-impact fracture
screening for people with specific medical conditions or treatments likely to cause osteoporosis (annual)
subsequent monitoring of low BMI
The decision whether to measure BMD in non-subsidised situations, and whether to treat a low BMD, may also be guided by an assessment of absolute fracture risk. The FRAX tool is commonly used, although somewhat controversially was developed by manufacturers of osteoporosis medication.7