Estimating Lumbar Spine Least Significant Change for Fewer than Four Vertebrae: The Manitoba BMD Registry

Introduction: The International Society of Clinical Densitometry recommends omitting lumbar vertebrae affected by structural artifact from spine BMD measurement. Since reporting fewer than 4 vertebrae reduces spine BMD precision, least significant change (LSC) needs to be adjusted upwards when reporting spine BMD change based on fewer than 4 vertebrae. Methodology: In order to simplify estimating LSC from combinations of vertebrae other than L1 -L4 (denoted LSCL1-4), we analyzed 879 DXA spine scan -pairs from the Manitoba BMD Program's ongoing precision evaluation. The additional impact on the LSC of performing the second scan on the same day vs different day was also assessed. Results: LSC progressively increased when fewer vertebrae were included, and also increased when the scans were performed on different days. We estimated that the LSCL1-4 should be adjusted upwards by 7 %, 24 % and 65 % to approximate the LSC for 3, 2, or 1 vertebral body, respectively. To additionally capture the greater LSC when the precision study was done on different days, LSCL1-4 derived from a precision study where scans were done on the same day should be adjusted upwards by 39 %, 60 % and 112 % for 3, 2, or 1 vertebral body, respectively. Conclusion: LSCL1-4 derived from a precision study where scans are performed on the same day can be used to estimate LSC for fewer than 4 vertebrae and for scans performed on different days.