Bone densitometry T and Z scores.Why do we have these confusing scores?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 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/cm 2. But when the bone density machines became commercial, the different companies would not agree on a standard measurement. A person would be about 6% higher on a Lunar machine than on a Hologic machine, even though both said they were reporting g/cm 2.
If the companies would have used the same standards, then we could always just look at the plain bone density in g/cm 2, just like we look at cholesterol in mg/dl or weight in kg. Unfortunately, that did not happen. Instead, the T-score was invented. T-scores are not used (to my knowledge) in any other aspect of clinical medicine, and for 20 years they have caused trouble and confusion.Some investigators have tried, unsuccessfully, to establish a 'standardized' unit of mg/cm 2. Equations have been published to convert Hologic, Lunar, or Norland measurements to standardized units. The NHANES study also reported the standardized units. The equations and converters are on the page about.The reference ranges are also problematic.
Rise’s relative scoring algorithm for the nth ranked player is calculated as follows, with 1 being the top rank: nth ranked player’s score = 100 - (n -1). 100/N The score will be a number between 0 and 100. The highest ranked player will have a score of 100 and the lowest ranked player will have a score of 100/N. 2 Advantages of Using Z-scores nClarity: The relationship between a raw score and the distribution of scores is much clearer. It is possible to get an idea of how good or bad a score is relative to the entire group.
Currently the NHANES study is used for the hip reference data by everybody. The different machines still use their own reference data sets for the spine because the spine was not included in the NHANES study.Z-scoresZ-scores can be used to compare a measurement to a reference value. The z-score is the number of standard deviations away from the average value of the reference group. This reference group usually consists of people of the same age and gender; sometimes race and weight are also included.This table shows how z-scores correspond to percentiles. The percentile is the percent of people in the population who have a lower bone density. A person who is average has a Z-score of zero and is at the 50th percentile. If the Z-score is -0.84 then 20% of people have a lower bone density.Pediatricians use percentiles to interpret the height of a child.
A child at the 5th percentile (same as Z-score of -1.65) is short for his or her age, and one at the 75th percentile is somewhat taller than average (Z-score of 0.68). The Z-score does not tell how tall a child is, because the average child gets taller as she gets older. On the other hand, if you know that a child is 40' tall, it does not mean anything unless you also know his age. You must know both the age and the percentile to know if this is a healthy height.For bone density, the Z-score will tell you if the bone density is close to the average value for the person's characteristics such as age, race and gender, but that still does not tell you if the bone is strong. Elderly white women have weak bones even if the bone density is average.Calculation of z-scoresYou need to have a table of reference values showing the mean (average) and standard deviation (SD) for the age, gender, race, skeletal site, and densitometer measurement units.
I call this the 'expected BMD'. The following table gives values from NHANES dataset.