So you have had a bone density scan and given a report with a T and Z score for hip and spine. The aim of this short tutorial is to help you better understand the results and what they mean for you.
According to Osteoporosis Australia the bone density scan – referred to as a DXA scan ( Dual Energy X-ray Absorptiometry) is really the ‘Gold Standard’ test for osteoporosis or osteopenia. 1
T Scores are a means of measuring bone loss compared to that of a 30 year old person of the same gender.
A T score of 0 means you have no bone loss compared to that of a 30 year old person of the same gender.
A T score of -1 indicates a bone loss of 1 standard deviation below that of a 30 year old healthy person of the same gender. So what does this one standard deviation really mean? It simply means a bone loss of 10%
For example a T score of -2 means a loss of 20% and -2.5 would mean 25%
A T score of -4 means a loss of 40% and this would be of some concern.
It is important to understand a T score of -1 and above is perfectly normal once over 50. A T score between -1 and -2.5 indicates lower than normal bone density for age – this is referred to as osteopenia. A score lower than -2.5 indicates significant bone loss – 25% or more and this is classified as osteoporosis.
To put things into perspective, it would be abnormal at age 50 or more, for example, to have a T score of 0. It would be as unusual as a 50 year old having not one wrinkle. A certain amount of bone loss is a perfectly normal process of ageing. It is important to understand this.
The Z score gives an indication of bone density compared to a healthy person of the same age and gender.
A Z score of -2 in a 50 year old would indicate a 20% greater loss of bone density compared to what the expected ‘normal’ bone loss for a 50 year old would be. A Z score of -2 is significant and requires investigation into the reason for bone loss other than normal aging.
DXA scans are usually done on what are considered the most vulnerable bones in the body. These are the lumbar spine and neck of femur. The wrist is also a vulnerable area and may be included in the test. Do not be concerned about the amount of radiation you could be exposed to during a DXA scan as it is far less than a normal chest x-ray.
It is useful to know that the small portable bone scan machines available in some pharmacies and travelling vans only measure the density of distal bones in the body such as the heel or finger. They can give some indication of bone density in these areas but they do not replace the gold standard test provided by a central scanning device in a hospital or private radiology practice. If you are concerned about a test result from one of these portable devices follow it up with your health professional.
Remember – some bone loss is perfectly normal. From age 30 when peak bone density occurs, there is a gradual decline in bone density as the percentage of bone resorption increases and bone formation decreases.
The major consequence of bone loss as we age is fracture. Having osteoporosis in itself does not mean you will have a fracture. If it is combined with significant muscle loss and poor balance (something known as sarcopenia) there is an increased risk of fracture.
Citations and Resources:
1. Osteoporosis Australia, Bone Density Testing in General Practice PDF
2. Osteoporosis Australia, Physiology of Bone Loss Radiology Clinic North Am. 2010 May:48 (3): 483-495