The International Society of Clinical Densitometry have published quality criteria for bone density measurements that should be fulfilled to ensure a high-quality scan has been performed from which recommendations for treatment can be taken. The general term for the criteria can be considered “concordance” i.e., agreement between two measurements.
For the World Health Authority diagnostic categories (Normal, Osteopenia, Osteoporosis) concordance between the hip and spine indicates bone density in both areas indicates the same diagnostic category: normal / normal, osteopenia / osteopenia, osteoporosis / osteoporosis. If there is a single category difference e.g., normal / osteopenia or osteopenia / osteoporosis, there is a minor discordance. If the difference is two categories i.e., normal / osteoporosis, the discordance is major. Major discordances are rare and should be
investigated as a matter of routine.
For the two hip and spine T scores, concordance means a difference of less than 1.0. Differences greater than 1.0 indicate a discordance and in the absence of any reasonable explanation for the difference, the results should be critically examined.
For WHO diagnostic criteria minor discordance, when the T scores are not discordant, the most common reason is because results lie on either side of the threshold dividing normal from osteopenia and osteopenia from osteoporosis.
For the spine, because multiple vertebrae are measured at the same time, further quality measures are needed. The first concerns the progressive increase in bone density between L1 and L4. L1 is the least loaded vertebra measured and will therefore have the lowest bone density, while L4 is the most loaded and should have the highest bone density. There should be progression from the lowest bone density at L1 to a higher level at L2, higher still at L3 and highest at L4. Failure of the bone densitometry to replicate this pattern could indicate a local abnormality within the lumbar spine that might need further investigation and
explanation or an inaccurately performed scan.
The third measure of concordance refers to the T scores of the four individual vertebrae measured. When considering the T score difference between adjacent vertebrae i.e., L1 & L2, L2 & L3, L3 & L4, it should be less than 1.0 at each level. If it is greater the vertebra that has the standout abnormal T score (be it too high or too low) should be omitted from the analysis of the overall spine T score.
Achieving all three quality criteria (hip and spine T score concordance, appropriate progression of bone density from L1 – L4, intervertebral T score concordance) indicates a well performed bone densitometry assessment irrespective of the technology used to measure the bone density. However, there are clinical circumstances when discordance could be appropriate for instance in a person who has had a stroke or spinal cord injury. People with these conditions do not have the same pattern of weight-bearing through the
spine and hips as people without the conditions and as a result there can be quite significant differences in the T scores between the hip and spine or within the spine itself. These variations may therefore be normal for that person.