Two commonly used fracture prediction tools – Garvan and FRAX (Australia) – have been found to underestimate the risk of osteoporotic fractures in Australian populations.
The calculators performed well in predicting hip fractures but poorly for major osteoporotic fractures (including spine, forearm and humerus), particularly in people with osteopenia or osteoporosis, according to investigators from Deakin University in Geelong, Victoria.
In a validation study involving 809 women and 821 men over the age of 50 enrolled in the Geelong Osteoporosis Study, they found that FRAXBMD predicted 52 major osteoporotic fractures in women but there were 115 observed. In men, the FRAXBMD predicted 26 major osteoporotic fractures and 73 were observed. Overall, FRAXBMD and FRAXnoBMD calculators underestimated the number of fractures by 54.8% and 46.1%, respectively.
The GarvanBMD tool predicted 139 fragility fractures in women and there were 184 observed, whereas in men it predicted 88 fragility fractures and there were 109 observed (non-significant difference). Overall the GarvanBMD and GarvannoBMD underestimated fragility fractures by 24.5% and 21.7% respectively.
Underestimation of fractures using both scores occurred primarily in individuals with osteopenia and osteoporosis.
Conversely, hip fractures were predicted well by both calculators in both men and women with a few exceptions – FRAXBMD and GarvannoBMD in women were both not accurate.
Despite the results, study co-author Associate Professor Nicholas Pocock from Department of Nuclear Medicine and Bone Densitometry, St Vincent’s Clinical School, Sydney, said the calculators were still useful tools.
“Certainly for hip fractures both FRAX and the Garvan perform well so I think that supports their use in the guidelines,” he told the limbic.
“They weren’t as obviously convincing for non-hip fractures. However, hip fractures are obviously the most important of the fractures and in that sense I think they are a useful tool to improve identification of people who will benefit from therapy.”
Professor Pocock said a possible explanation of the underestimation of fractures by FRAX could be its design, which adjusts fracture risk based on expected mortality rates, resulting in a decline of absolute risk for those aged over 80 years. This would result in a lower calculated fracture risk in the elderly population than would be predicted based on BMD and risk factors independent of expected mortality.
What’s more, where data regarding the rates of major osteoporotic and hip fractures are unavailable, the Australian FRAX calculator assumes that the age- and sex- specific pattern of these fractures is similar to that observed in Sweden. Professor Pocock argues that this assumption may not be correct because of Australia’s multicultural nature.
“The problem with a country like Australia, which is very multicultural, is that the absolute fracture risk varies tremendously depending on your country of origin as well as within Australia – the cultural signifiers in Sydney is going to be very different to rural towns for instance. So our study does highlight the need for collection of national data to improve risk calculators.”
But acknowledging the limitations of fracture-risk calculators Professor Pocock said a focus on targeting patients after a first minimal-trauma fracture and getting them onto anti fracture treatment would have a much bigger effect on reducing osteoporotic fracture rates than trying to predict which lower-risk patients are likely to benefit from anti-osteoporotic treatment.
In Australia he said there was a significant treatment gap between these individuals at high-risk and those receiving treatment, where fewer than 30% are being treated with anti-fracture therapies. This was also true for the most serious osteoporotic fracture, hip fracture.
“What we need to keep in the back of our mind is that these risk calculators are a tool that we use alongside bone density measurement and other clinical parameters to make treatment decisions. That’s important particularly in the presence of minimal trauma fracture in people over the age of 50 – the single biggest at-risk group being missed.”
The findings are published in Osteoporosis International.