Interpreting bone mineral density (BMD) in premenopausal women

Bone mineral density (BMD) measurement is currently the best method of identifying those at risk of osteoporotic fractures and should be carried out on individuals with any of the risk factors. The lower the BMD the higher the risk for fracture is.

The results of the scans are plotted against the normal range for age and the BMD score can be seen as a (+). This allows one to compare the individual's BMD to the average BMD of an age and sex matched group. The BMD scores can be expressed in terms of standard deviations (SD) from the mean bone density of young people (T- score) or an age matched population (Z-score).

World Health Organization (WHO) defines osteoporosis when BMD is 2.5 standard deviations below the mean value for young women (between age 20 and 40 years). This is defined as a T-score of -2.5 and below. This classification of BMD for the diagnosis of osteoporosis is based on extensive data in postmenopausal white women. The commonest mistake in practice which causes great anxiety in many young women is that this criterion is often used for the diagnosis of osteoporosis in premenopausal women.

The WHO classification of osteoporosis (T-score of -2.5 and below) should not be applied to healthy premenopausal women (age 20 years to menopause). For premenopausal women Z-score should be used when interpreting BMD results. Any Z-score up to -2.0 should be regarded as normal and a Z-score lower than -2.0 should be interpreted as "lower than expected for age". One should avoid making the diagnosis of osteoporosis in young women unless, in addition to lower than expected for age BMD, other factors like fragility fractures, long- term use of known bone unfriendly medications or a disease known to affect bone health are present.