Standard Scores on Bone Density Reports

T-scores and z-scores are found on the computer-generated bone densitometry printouts from virtually every manufacturer of bone density equipment. Figure 3-2 is the printout from a spine bone density study performed on an older Lunar DPX device. The individual BMD values for each vertebra are listed as well as the BMD values for each possible combination of contiguous vertebrae. The two columns adjacent to the BMD values reflect z-scores. One column is entitled "young-adult z" and the other, "age-matched z." Based on an understanding of the z-score, it is clear that these z-scores indicate how many standard deviations above or below the mean value, the patient's BMD value lies. But what mean value and SD were used to calculate these z-scores? The young-adult z-score was calculated using the average peak bone density and SD for the young adult. The column entitled age-matched z reflects the use of the average bone density that would have been predicted on the basis of the patient's age. In this case, the L2-4 BMD average of 1.288 g/cm2 has a young-adult z-score of 0.73 and an age-matched z-score of 0.92. This means that the L2-4 BMD is 0.73 standard deviations above the average peak bone density of the young adult and 0.92 standard deviations above the BMD that would have been predicted on the basis of the patient's age.

Figure 3-3 is a bone density printout from a spine study performed on a Hologic QDR-4500. The BMD values for each individual vertebra and the average BMD value for L1-4 are evident. Adjacent to these values, two of the four columns are now entitled

Score Bone Density Test

Fig. 3-2. A Lunar DPX PA spine study in which the standard scores are presented as young-adult z-scores and age-matched z-scores, reflecting the older, albeit, correct terminology for the standard score comparisons. The standard scores and BMD are listed for each vertebra and every possible combination of contiguous vertebrae.

Fig. 3-2. A Lunar DPX PA spine study in which the standard scores are presented as young-adult z-scores and age-matched z-scores, reflecting the older, albeit, correct terminology for the standard score comparisons. The standard scores and BMD are listed for each vertebra and every possible combination of contiguous vertebrae.

simply 'T-score" and "z-score." There is nothing in the title of these columns to definitely indicate which average value is being used to calculate these standard scores. The "30.0" in parentheses next to the "T" however, indicates that the comparison is to the average bone density of a 30-year-old. Figure 3-4 is a Norland DXA PA spine study. A T-score of -0.5 and z-score of 0.72 are noted for the L2-L4 average BMD. When the formats used in these Hologic and Norland DXA studies are employed, the average value used to calculate the T-score is always the average peak BMD of the young adult. The average value used to calculate the z-score is always the average age-matched BMD. It is obvious, however, that the values in the T-score column do not look like true T-scores. T-scores, after all, should be scores like 30 or 75. The use of either a + or - sign to indicate the position relative to the mean value is not required with the traditional T-score. These values in the T-score column actually look more like z-scores. And in fact, that is exactly what they are. These are z-scores that, in bone densitometry only, are renamed T-scores. This allows one to understand without so stating that the reference average in use here is the young-adult peak BMD and that the reference average in use for the calculation of the z-score is the age-matched BMD. Therefore, the T-score of -3.33 for the L1-L4 average BMD seen on the Hologic QDR 4500 study means that the L1-L4 average BMD is 3.33 standard deviations below the average peak BMD of the young adult. This convention of renaming the young-adult z-score the T-score has been increasingly adopted by all manufacturers of bone density equipment and has also found its way into the bone density literature. In Fig. 3-5, a Lunar Prodigy PA spine study reflects the use of the T- and z-score terminology, although the abbreviations YA and AM for young adult and age-matched still remain. It is important to remember that the young-adult z-score is identical to the T-score in this context. The age-matched z-score is identical to what is now simply called the z-score.

Score Score

Fig. 3-3. A Hologic QDR 4500 PA spine study in which the standard scores are presented using the terminology of T-score and z-score, rather than young-adult z-score and age-matched z-score. This terminology has become the accepted format for standard score comparisons. These values, in addition to the BMD, are given for each vertebra and for the contiguous vertebrae of L1-L4. Study provided courtesy of Dr. Paul Miller, Colorado Center for Bone Research, Lakewood, CO.

Fig. 3-3. A Hologic QDR 4500 PA spine study in which the standard scores are presented using the terminology of T-score and z-score, rather than young-adult z-score and age-matched z-score. This terminology has become the accepted format for standard score comparisons. These values, in addition to the BMD, are given for each vertebra and for the contiguous vertebrae of L1-L4. Study provided courtesy of Dr. Paul Miller, Colorado Center for Bone Research, Lakewood, CO.

Bmd Score Report
Fig. 3-4. A Norland XR-36 DXA PA spine study. The T-score and z-score are given for the L2-L4 BMD. Study courtesy of Norland, a CooperSurgical Company, Ft. Atkinson, WI.
Bone Density Score
Fig. 3-5. A Lunar Prodigy DXA PA spine study. The older terminology of young-adult z-score and age-matched z-score has been replaced with young-adult (YA) TT-score and age-matched (AM) z-score.

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