T scores and z scores

A 79 year old white woman had her bone mineral density (BMD) measured to assess for osteoporosis and fracture risk. The table shows the results of dual energy x ray absorptiometry (DXA).

Region	Bone mineral density (g/cm2 )	T score	z score
Right hip
Femoral neck	0.607	-2.2	0.1
Trochanteric	0.661	-0.4	1.3
Intertrochanteric	1.001	-0.6	1.2
Total	0.841	-0.8	1.2
Lumbar spine
L1	0.782	-1.3	1.0
L2	1.083	0.5	3.1
L3	1.179	0.9	3.6
L4	1.218	0.9	3.8
Total	1.070	0.2	2.9

Which of the following statements, if any, are true?

a) The z scores enable BMD measurements to be compared between men and women of different ages and ethnic groups

b) The z scores are normally distributed

c) This woman's BMD measurements were below the mean for her age and ethnicity

d) The T scores compare the woman's BMD measurements with those of white women at an age when bone density is at its peak

Answers

Answers a , b , and d are true, whereas c is false.

Measurement of BMD in the hip and spine is the gold standard for the diagnosis of osteoporosis and assessment of fracture risk. BMD is the ratio of bone mineral content to bone area (expressed in g/cm²). Measurements for three regions of the woman's right hip are given-the femoral neck, tronchanteric region, and intertrochanteric region. A total BMD measurement was obtained by scanning the entire hip region. The lumbar spine was also scanned, and BMD measurements for the vertebrae L1 to L4 are shown. A total BMD measurement was obtained for the...