At the first follow-up, the patient's BMD, based on a DXA scan, was up 5.3%. However, at the second follow-up, his BMD had dropped by 5.1%. His primary care physician referred the man to determine why he was no longer responding to therapy.
“What is the first thing to do?” asked Dr. Michael Lewiecki, director of the New Mexico Clinical Research & Osteoporosis Center in Albuquerque. Should you “evaluate for adherence to therapy? Ask about calcium and vitamin D intake? Order lab tests to evaluate for factors contributing to bone loss?”
No, the first step is to actually look at the DXA image, said Dr. Lewiecki. In this case, the vertebral bodies were mislabeled on the second follow-up DXA scan.
Typically, the DXA computer program selects the labeling for vertebral bodies on DXA scans. It's not uncommon for mislabeling to occur though, said Dr. Lewiecki. A technologist goes over the labeling to double check the computer. Finally, the image should be reviewed by a physician interpreter as an additional check before the report is generated. In this case, the incorrectly labeled scan was not caught and false BMD measurements were reported to the ordering physician.
“When you have a situation in which BMD changes unexpectedly, it is important to verify that it is a valid comparison,” said Dr. Lewiecki.
Sometimes a left hip is incorrectly compared with the right hip or vice versa when assessing BMD over time. Other times, the lines used by the computer to delineate the edges of bone are misplaced, which can have a big effect on the amount of bone measured.
“Make sure you're comparing apples to apples,” he said. Reanalysis of the image with vertebral bodies correctly labeled showed that the patient's BMD for L1-L4 had been stable since the first follow-up. Response to therapy actually was good, requiring no change in alendronate and hydrochlorothiazide therapy and no laboratory testing for nonresponse to therapy.
Baseline DXA scan for L1-L4 (left): DXA at 1-year follow-up shows a 5.3% increase in BMD (center). However, a year later, mislabeling of vertebral bodies showed a 5.1% decrease in BMD (right). The erroneous labeling was clear when compared with the previous DXA images. Photos courtesy Dr. Michael Lewiecki
