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Date of release: 08 March, 2010

Differences in female bone density reference ranges between countries


Bone mineral density (BMD) measurement using dual-energy X-ray densitometry (DXA) has long been established as the gold standard in the screening of osteoporosis. BMD measurement provides two indices: the Z-score and the T-score. The Z-score is the number of standard deviations that the individual measurement differs from the mean value of the healthy population, matched for race, age and sex. Accordingly, the T-score is the number of standard deviations that the individual measurement differs from the mean peak bone density of the healthy young population, matched for race and sex. The manufacturers, when setting normative data, take into account three races, meaning the Caucasian, Black and Asian populations. It is becoming apparent, however, that, within the same race, there are significant differences across different countries. This important issue is highlighted in the article by Noon and colleagues in the latest issue of Osteoporosis International [1]. The authors have assessed the Z-scores of women participating in three different clinical studies conducted in the UK, using the manufacturer’s US normative data. In all three studies, the mean Z-scores were above 0, meaning that the normal BMD of the UK women is higher than that of the US women.

Comment

Several epidemiological studies so far have highlighted the wide variation of physiological BMD values across different populations, within the same race. In the EVOS study, which evaluated the prevalence and risk factors of vertebral osteoporosis in Europe, variations of BMD as high as one standard deviation were detected across participating countries [2]. The same geographic variation was recently reported in the NEMO study, which aimed to evaluate country-specific differences in normal values of spine and hip BMD in European men and women [3]. The results of these studies stress the importance of establishing national normative data. Most laboratories use the normative data supplied by the manufacturer, which are usually based on the US population. Consequently, European women are evaluated according to the US normative data. This may lead either to over-diagnosis of osteopenia and osteoporosis or to loss of cases. Both options increase socioeconomic cost, either by over-treatment or by failure to prevent an osteoporotic fracture. DEXA software, therefore, should be equipped with country-specific normative data when calculating the T-score. Alternatively, each country should establish a population-specific correction coefficient for adjusting US normative data to the local population.
 
Individual osteoporotic fracture risk can be estimated by the FRAX score [4], a country-specific algorithm which is available on the internet. The FRAX score takes into account hip bone mineral density in g/cm2 (not T-scores), together with other risk factors for osteoporosis, and calculates the 10-year country-specific risk for a hip or major osteoporotic fracture.

Comentario

George Creatsas
2nd Department of Obstetrics and Gynecology, Aretaieion Hospital, Athens, Greece

    References

  1. Noon E, Singh S, Cuzick J, et al. Significant differences in UK and US female bone density reference ranges. Osteoporosis Int 2010; January 9. E-pub ahead of print.
    http://www.ncbi.nlm.nih.gov/pubmed/20063090

  2. Lunt M, Felsenberg D, Adams J, et al. Population-based geographic variations in DXA bone density in Europe: the EVOS Study. European Vertebral Osteoporosis. Osteoporosis Int 1997;7:175-89.
    http://www.ncbi.nlm.nih.gov/pubmed/9205628

  3. Kaptoge S, da Silva JA, Brixen K, et al. Geographical variation in DXA bone mineral density in young European men and women. Results from the Network in Europe on Male Osteoporosis (NEMO) study. Bone 2008;43:332-9.
    http://www.ncbi.nlm.nih.gov/pubmed/18519175