A One-Woman Dexa Scan Report

Elaine Mansfield, July 27, 2004

Bone scans give us valuable information and let us know we have bone density problems before we suffer irreparable damage. On the other hand, dexa scan measurement may not be quite as accurate as we might hope it to be. If we know more about the accuracy of the testing, we know how to interpret the results better

Below, you'll find my dexa scan results. I had two tests taken two weeks apart with the same equipment and operator. One year later, I was tested on different equipment, but again two tests were taken on this equipment, two weeks apart with the same operator. In tests taken two weeks apart gave readings as much as 5.7% different. I believe we need to see long term trends with quite a few readings before getting excited about small percentage changes in bone density.

My 2003 and 2004 measurements were taken on different machines, so I also compared the two tests using the formulas recommended for doctors. The discussion of that comes next, followed by my conclusions.

Norland dexa scan equipment

Date measured:

7/03/03 L2-L4: .848 g/cm2
7/15/03 L2-4: .802 g/cm2, T score –1.85

–5.7% difference in tests taken 2 weeks apart

7/03/03 L femoral neck: .641 g/cm
7/15/03 L femoral neck: .631 g/cm2, T score –3.04

-1.5% in tests 2 weeks apart

Lunar dexa scan equipment

Date measured:

7/13/04 L2-4: .914 g/cm2, T score –2.4
7/26/04 L2-4: .925 g/cm2, T score –2.3

+1.2% difference in tests 2 weeks apart

7/13/04 Femoral neck average: .772 g/cm2 or T score –1.8
7/26/04 Femoral neck average: .735 g/cm2, T score –2.0

-4.8% difference in tests taken 2 weeks apart

I went to the following website for the standard formulas used for comparing results from different dexa equipment: http://courses.washington.edu/bonephys/opBMDs.html (Y.Lu, T.Fuerst, S.Hui and H.K.Genant, Standardization of Bone Mineral Density at Femoral Neck, Trochanter and Ward's Triangle, Osteoporos Int (2001)12:438-444. and Hui, SL, et al. Universal Standardization of Bone Density Measurements: A Method with Optimal Properties for Calibration Among Several Instruments. J Bone Mineral Research 1997;12:1463-70.)

I plugged in my test results from 7/15/03 and 7/13/04 and the comparison between the Norland data and Lunar data showed a –16.5% in the spine and +12% in the L hip in one year—impossible results since bone density increases only 1-2% a year in post menopausal women.

My Conclusions:

1. The official medical formulas for comparing results from different machines are unreliable—at least in my case.

2. Bone density testing on the same machine and operator, same time of day, 2 weeks apart, gave a range from -1.5 to -5.7% difference one year and +1.2 to -4.8% the next. An article in the Journal of Bone and Mineral Research reported 4% variation in bone scans taken within 7 days, so these findings might be common if women were to do repeat testing. (Neither dexa scan operator had ever done a repeat test, so they were both surprised by the discrepancy in the results.) For individual results as opposed to a group study where individual variations are averaged, I’ll look for long-term trends of 6-7% change over a number of years before celebrating or lamenting dexa scan results.

3. Research is clear that strengthening exercise diminishes fracture risk even when bone scan measurement is unchanged. A dramatic example from the journal Bone: in a 2002 study, a 1/3 lower risk for spinal fracture resulted from a two-year back-strengthening program even though bone scan measurements didn’t improve in exercisers compared with controls. Actual fractures were assessed 8 years after the exercise study ended, and exercisers had not continued the back exercises after the initial 2-years.