The current presentation is a brief overview of precision and follow up scans in BMD with especial attention to least significant change and Z-score changes in children

1. PRECISION AND FOLLOW
UP SCANS IN BONE
DENSITOMETRY
Ramin Sadeghi

2. Precision Assessment
• Each DXA facility should determine its precision error
and calculate the LSC.
• The precision error supplied by the manufacturer
should not be used.
• If a DXA facility has more than one technologist, an
average precision error combining data from all
technologists should be used to establish precision
error and LSC for the facility, provided the precision
error for each technologist is within a pre-established
range of acceptable performance.

3. Precision Assessment
• Each technologist should do one complete precision
assessment after basic scanning skills have been
learned (e.g., manufacturer training) and after having
performed approximately 100 patient-scans.
• A repeat precision assessment should be done if a
new DXA system is installed.
• A repeat precision assessment should be done if a
technologist’s skill level has changed.

4. Precision Assessment
• To perform a precision analysis:Measure 15 patients 3
times, or 30 patients 2 times, repositioning the patient
after each scan.
• Calculate the root mean square standard deviation (RMS-SD)
for the group.
• Calculate LSC for the group at 95% confidence interval.

5. Precision Assessment
• The minimum acceptable precision for an individual
technologist is:
• Lumbar Spine: 1.9% (LSC=5.3%)
• Total Hip: 1.8% (LSC=5.0%)
• Femoral Neck: 2.5% (LSC=6.9%)
• Retraining is required if a technologist’s precision is worse
than these values.

6. Precision Assessment
• Precision assessment should be standard clinical
practice. Precision assessment is not research and
may potentially benefit patients.
• It should not require approval of an institutional
review board.
• Adherence to local radiologic safety regulations is
necessary. Performance of a precision assessment
requires the consent of participating patients.

7. BMD Comparison Between Facilities
• It is not possible to quantitatively compare BMD or to
calculate a LSC between facilities without cross-
calibration.
• Patients should return to the same DXA device that
was used to perform their most recent prior study,
provided that the facility in vivo precision and LSC
values are known and do not exceed established
maximum values.

8. Serial BMD Measurements
• Serial BMD testing in combination with clinical
assessment of fracture risk, bone turnover markers,
and other factors including height loss and trabecular
bone score, can be used to determine whether
treatment should be initiated in untreated patients,
according to locally applicable guidelines.
• Serial BMD testing can monitor response to therapy
by finding an increase or stability of bone density.

9. Serial BMD Measurements
• Serial BMD testing should be used to monitor
individuals following cessation of osteoporosis
pharmacologic therapy.
• Serial BMD testing can detect loss of bone density,
indicating the need for assessment of treatment
adherence, evaluation of secondary causes of
osteoporosis, and re-evaluation of treatment options.

10. Serial BMD Measurements
• Follow-up BMD testing should be done when the
results are likely to influence patient management.
• Intervals between BMD testing should be determined
according to each patient’s clinical status: typically
one year after initiation or change of therapy is
appropriate, with longer intervals once therapeutic
effect is established.
• In conditions associated with rapid bone loss, such
as glucocorticoid therapy, testing more frequently is
appropriate.

11. WHICH SKELETAL SITES SHOULD BE
USED FOR MONITORING?
• 1. Measure the skeletal site or type of bone (trabecular or
cortical) that you expect to be affected.
• 2. Of the sites potentially affected, measure the site at
which the greatest change in BMD is expected.
• 3. Of the sites potentially affected, measure the site at
which you can measure the BMD with the best precision.
• 4. Peripheral sites are not used for monitoring by any
technique.

12. WHICH SKELETAL SITES SHOULD BE
USED FOR MONITORING?
• In a position statement from the International Society for
Clinical Densitometry (ISCD) published in 2002, the PA
lumbar spine was described as the preferred site for
monitoring.
• The total femur region of interest was an alternate choice
when the PA spine could not be measured for any reason.
• In recommending the PA lumbar spine as the preferred
choice, the ISCD panel members noted that the PA
lumbar spine provided the best combination of magnitude
of change and precision.

13. WHICH SKELETAL SITES SHOULD BE
USED FOR MONITORING?
• The total femur was recommended by ISCD because of
its greater area in the proximal femur. This does indeed
result in excellent precision at the total femur.
• Rates of change at the total femur tend to be slow. They
are often less than those seen in the femoral neck, a
region with a much smaller area and certainly less than
those seen in the trochanteric region. This slower rate of
change is, in part, offset by the excellent precision,
enabling the physician to detect a significant change in
bone density within a reasonable period of time.

14. Follow-Up DXA Report
• Statement regarding which previous or baseline study and
ROI is being used for comparison
• Statement about the LSC at your facility and the statistical
significance of the comparison
• Report significant change, if any, between the current and
previous study or studies in g/cm2 and percentage.
• Comments on any outside study including manufacturer
and model on which previous studies were performed and
the appropriateness of the comparison
• Recommendations for the necessity and timing of the next
BMD study

15. Clinical examples

16. Case 1
• A patient with nephrotic syndrome receiving
corticosteroid.
• Follow up BMD on a same device

19. Case 2
• A patient with diagnosis of osteoporosis receiving
bisphosphonate therapy

21. Case 3
• A follow up scan using Norland device

23. LSC=2.77*C.V/100

26. % change
in lumbar
spine=
0.79*0.8%=
0.00632
0.00632<0.025
statistically non
significant

27. Case 4
• A pediatric patient with trichothiodystrophy

32. Pediatric serial BMD
• Serial DXA reports should include the same
information as for baseline testing. Additionally,
indications for follow-up scan, technical
comparability of studies, changes in height and
weight, and change in BMC and areal BMD Z-scores
should be reported.
• Z-score change is important not change in BMD