Bone Densitometry Relative to Normal Young Adult (T Score)

 

Common sites for measurement of bone mineral density:

(1) femoral neck (best site for predicting hip fractures)

(2) vertebra (best site for predicting spinal fractures)

(3) radius

(4) calcaneus

 

Parameters:

(1) mean bone density for a normal adult 29 to 50 years of age in g per square cm

(2) standard deviation (SD) in the bone density for a normal adult 29 to 50 years of age

(3) patient's bone density in g per square cm

(4) history of fragility fractures

 

T score =

= number of standard deviations above or below the mean bone mineral density for a normal adult 29-50 years of age =

= ((patient bone mineral density in g/cm^2) - (mean bone mineral density for normal adult 29-50 years of age)) / (standard deviation for normal adult 29-50 years of age)

 

Interpretation:

• The lower the T score the lower the bone density.

• The risk of bone fracture increases for each SD below the mean. It increases exponentially as the T score decreases below -2 (2 SD below the mean).

T Score	History of Fragility Fracture	Interpretation
> - 1.0	NA	normal
-1.0 to -2.5	NA	osteopenia
< -2.5	none	osteoporosis
< -2.5	1 or more	severe osteoporosis

 

Comparison with the Z-score:

(1) The Z score is similar but the reference population consists of adults matched for age and gender.

(2) Unlike the T score the mean and standard deviation for an older adult may change each year.

(3) Since the mean bone density of older adults declines with age, the Z score may be less effective in indicating the absolute loss of bone density. A different Z score cutoff for each age is required for interpretation.