Bone Density Test Results
Bone density test results involve a T-score and a Z-score. While seeming sophisticated and statistical, bone scans may have accuracy or reliability issues, especially in comparison to other scans. Your bone density test results can be different based on the bones measured, the equipment used, or even how the technician tests you. Wnen possible, try to use the same, measurement equipment and technicians to eliminate design variability.
According to the NYTimes.com:
The T score compares your bone density with that of healthy young women. The Z score compares your bone density with that of other people of your age, gender, and race. In either score, a negative number means you have thinner bones than the standard. The more negative the number, the higher your risk of a bone fracture. Bone mineral density testing does not diagnose fractures.
The University of Washington provides a technical discussion of bone density test results (http://courses.washington.edu/bonephys/opbmdtz.html):
The T-score is the number of standard deviations below the average for a young adult at peak bone density. There are different T-scores depending on which group of young adults was used as the reference (for example, Caucasian women, Hispanic men). The Z-score is the number of standard deviations below an average person of the same age. There are also different Z-scores depending on the group used as a reference. Furthermore, a person can have one T-score at the femoral neck, another at the total hip, and another at the spine.
One may consider the severity of fracture of a given bone when evaluating bone density test results. For example, a hip or spinal fracture can immobilze an individual. In that respect, they are much more serious than, say, a wrist fracture.
The World Health Organization (WHO) has defined the following categories based… on bone density test results… in white women:
• Normal bone: T-score better than -1
• Osteopenia: T-score between -1 and -2.5
• Osteoporosis: T-score less than -2.5
• Established (severe) osteoporosis
includes the presence of a non-traumatic fracture.
The WHO committee did not have enough data to create definitions for men or other ethnic groups. One standard deviation is at the 16th percentile, so by definition 16% of young women have osteopenia! As shown below, by the time women reach age 80, very few are considered normal.
It is important to realize that the T-score alone does not predict fractures, and osteopenia is not a disease. The early bone density machines in the 1970's and early 1980's all used different kinds of units, so results were reported in Z-scores to allow comparisons to normal people. Later bone density was measured in large populations and the Z-scores [from bone density test results] were compared to the general population and not just to healthy people.
In the 1990's most people were using DEXA machines, which report units in g/cm2. But when the bone density machines became commercial, the different companies would not agree on a standard measurement. If the companies would have used the same standards, then we could always just look at the plain bone density in g/cm2, just like we look at cholesterol in mg/dl or weight in kg. Unfortunately, that did not happen. Instead, the T-score was invented. …For 20 years they have caused trouble and confusion.
Because of T-score variability, there are things you can do to monitor your bone health from test to test. This may be important if you have switched calcium supplements (such as to AdvaCAL) and made lifestyle/diet changes to improve bone strength. For consistency, try to use the same bone density equipment, the same skeletal measures and even the same technician if possible, Ask your doctor for guidance. Then you can readily compare current to previous bone density test results.