A One-Woman Dexa Scan Report: 2004
Bone scans let us know we have bone density problems before we suffer irreparable damage, but dexa scan measurement may not be as accurate as we expect. If we know more about the accuracy of the testing, we know how to interpret the results.
Below, you’ll find my dexa scan results. I had two tests taken two weeks apart with the same equipment and operator in 2003. One year later, I was tested on different equipment. I had two tests on this equipment, two weeks apart with the same operator. Tests taken two weeks apart gave readings differing as much as 5.7%. With this in mind, we need to see long-term trends with quite a few readings before getting excited about small percentage changes in bone density.
Norland dexa scan equipment
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% difference in tests 2 weeks apart
Lunar dexa scan equipment
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
My 2003 and 2004 measurements were taken on different machines.
I went to the Osteoporosis and Bone Physiology website for the standard formulas used for comparing results from different dexa equipment. I plugged in my test results from 2003 and 2004 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.
The official medical formulas for comparing results from different machines are unreliable—at least in my case.
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. (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.)
Exercise that strengthens muscles diminishes fracture risk even when bone scan measurement is unchanged. A dramatic example of this comes from the journal Bone. In a 2002 study, spinal fracture was lowered by two-thirds after 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, so the improvement in fracture risk lasted.