How should GPs use bone densitometry?
Selecting patients for DXA scan
Age, prior fracture and BMD are the most powerful predictors of future fracture risk. Consider a DXA scan in any patient with risk factors for osteoporosis in whom knowledge of BMD may influence management. Your local DXA service is likely to have locally agreed referral criteria to follow and there is also advice from bodies such as NICE (guidance on osteoporosis), SIGN (management of osteoporosis) and the Royal College of Physicians (corticosteroid guidance).
It may be helpful to use the FRAX® tool to estimate the patient's 10-year fracture probability and the associated NOGG guidance to decide whether DXA referral would be helpful. In Scotland, GPs should refer to the SIGN guidance which recommends using QFracture or FRAX® as the fracture risk assessment tool and referral for DXA when the 10 year risk exceeds 10%. Even if the fracture risk is very high it is helpful to know BMD in order to assess how well the patient is likely to respond to drug therapy and as a baseline to monitor progress. In most cases, it is clinically appropriate and feasible to send a patient over the age of 75 years for a DXA scan. When following SIGN guidance, it is recommended that all people with a history of fragility fractures over the age of 50 should be offered DXA scanning to evaluate the need for antiosteoporosis therapy.
When referring a patient for a DXA scan, the NOS leaflet Scans and tests may be helpful.
How should I interpret DXA scans?
Most DXA services provide information about the interpretation of the BMD result for an individual patient. If this is not available, it is important to remember that there are other risk factors for fracture that should also be taken into account when interpreting the BMD result. BMD of the femoral neck can be entered into the FRAX® probability assessment for a patient who has not already been treated for osteoporosis.
Good practice for reporting DXA scans is outlined in the NOS Practical Guide Reporting Dual Energy Absorbtiometry Scans in adult fracture risk assessment.
Repeating DXA scans in patients at risk of osteoporosis
The decision as to whether and when to repeat a DXA scan will depend on the initial results and the individual patient's circumstances. Factors to take into account are whether the patient has risk factors for accelerated bone loss (such as corticosteroid therapy) or medical conditions or treatment predisposing the patient to bone loss (such as inflammatory disease, malabsorption, aromatase inhibitor therapy or recent menopause). It is rarely helpful to repeat DXA scans within two years. Repeat measurements should be guided by local service agreements and the advice given at the time of the baseline scan.
Using follow-up scans to monitor treatment
There is a lack of consensus on this issue. DXA scans are of limited value in assessing response to treatment and a treatment response can rarely be demonstrated in less than 2 years. This is because the changes in BMD in response to treatment are small and slow and similar in magnitude to the error of the DXA measurement. However, repeat measurement of BMD can provide positive reinforcement to encourage continued compliance with treatment and is helpful in identifying patients who are not responding adequately to treatment.