How is Osteoporosis diagnosed?

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The diagnosis of Osteoporosis is simple.  If a person suffers a minimal trauma fracture then by definition they have Osteoporosis.

Unfortunately, that fact is often overlooked.

For example, there are many patients that present with a fracture, such as a middle aged female who falls and breaks her wrist, that are treated with immobilisation in a plaster cast with/without surgical stabilization.  Sadly however their osteoporosis is not considered and hence not treated.

Consequently, they sustain another fracture at some point in the future, since the greatest risk of having a fracture is having had a fracture in the past.   This represents a missed opportunity, since with treatment of the underlying cause (that being the osteoporosis) may have prevented that second fracture.

Therefore, it is quite appropriate that a clinician may request an x-ray to evaluate for a vertebral fracture that the patient may not be aware of.

It should be noted though, that the diagnosis of Osteoporosis can be made in those who have not yet had a minimal trauma fracture.  The challenge historically was how to identify the condition prior to a fracture occurring during which it does not cause any symptoms.  The breakthrough came with the ability to measure bone mass.  This revolutionized the approach to the diagnosis of Osteoporosis.

The technology was called bone densitometry (BMD) and involves passing a low dose of radiation through the skeleton in order to measure the amount that is absorbed.  This directly relates to the density of bone.  The best way to measure this uses a technology called dual energy X-ray absorptiometry or DXA. The lumbar spine and the neck of the femur (thigh bone) are the sites where bone density is measured and occasionally the distal (furthest) part of the forearm is also assessed.

Bone density refers to the amount of bone tissue for a given unit of area, which BMD measures as grams per square centimeter (g/cm2).  Although this value is important, interpretation of its meaning was difficult.  As a result a group of experts came together to devise a method that would allow clinicians to understand the result easily and use the information in a reliable way for diagnosis and management decisions.  They proposed that bone density of an individual be compared to a standard.  They chose two standards.

The first standard is the average bone density for a population of the young, which they termed the T-score, and the second compares to a population of similar age, termed the Z-score.  Furthermore, it was acknowledged that bone density would vary between ethnic groups and as such the BMD of an individual should be compared to those of a similar racial background.  The T-score gives an overall impression of bone density and hence predict the risk of fracture.  The Z-score is used to determine whether the bone density is at a level expected for the persons age, which if low may suggest a further cause of bone loss apart from age alone.

The T-score and Z-score are measured as standard deviations away from the average (termed the mean).  This may be a difficult concept to grasp for those without a background in mathematics (read blog post on making DXA more easy to understand) since a standard deviation is a statistical term that generally relates to a parameter that has a normal distribution.  There are numerous examples of measures that have a normal distribution, such as a person’s height, weight, and obviously bone density.  When such parameters are measured in a population most people would be close to the average, albeit slightly higher or lower.   Two thirds of the population would be within one standard deviation from the mean, and only 5% would be more than two standard deviations away.  Therefore a T-score of -2.0 would mean that the bone density is 2 standard deviations below the mean, at which level only about 2.5% of the ‘normal’ population would have a similar result.

Interestingly, on the basis of this, the group of experts who represented the World Health Organisation (WHO) decided that the diagnosis of Osteoporosis can be made according to the BMD when the T-score is below -2.5.  However, they highlighted that a T-score below -1.0 is also abnormal and as such they assigned the term Osteopaenia when the T-score is between -1.0 and -2.5.  Subsequent research has validated these values as predictors of future risk of fracture.

However, BMD is only one factor that goes towards predicting the future risk of fracture.   Quantitating this risk more accurately has been a particular topic of research interest.

Recently, an algorithm has been published by WHO to achieve this aim that they have termed FRAX, which is an acronym for Fracture Risk Assessment tool.

Their website describes it best by stating that it is ‘based on individual patient models that integrate the risks associated with clinical risk factors as well as bone mineral density (BMD) at the femoral neck’. The FRAX® models have been developed from studying population-based cohorts from Europe, North America, Asia and Australia.  The FRAX® algorithms give the 10-year probability of fracture, which includes a 10-year probability of hip fracture and the 10-year probability of a major osteoporotic fracture (clinical spine, forearm, hip or shoulder fracture).   This is already being used by clinicians in their management decisions and in time, probably, governments in their decisions about subsidising therapy.

Once the diagnosis of Osteoporosis is made, the clinician is likely to order other investigations to assist in their approach to managing the disease.  These are most commonly blood and urine tests.  These may include measurement of Vitamin D, Calcium, Parathyroid hormone, Thyroid function and renal function.  It should be noted that this list is by no means exhaustive.