Osteoporosis and Bone Densitometry Measurements by Giuseppe Guglielmi

Author:Giuseppe Guglielmi
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

3.3 Precision and Accuracy

Precision measures the reproducibility of a bone densitometry technique, and is usually expressed as a coefficient of variation (CV) or standardised CV, which takes into account the range of measurements of the particular method (Gluer et al. 1995). To be clinically useful the precision needs ideally to be in the region of 1 %, and certainly better than 3 %. The precision for total hip and lumbar spine is approximately 1 %; for femoral neck and trochanter CV it is 2.5 % and for Ward’s area it is 2.5–5 %. In peripheral sites precision is 1 % in the distal radius, 2.5 % in the ultra-distal radius and 1.4 % in the calcaneus (Grampp et al. 1993; Pacheco et al. 2002; Shepherd et al. 2006). The measurement sites generally used in clinical diagnosis, in contrast to research studies, are therefore lumbar spine (L1–4), femoral neck and total hip (Kanis and Gluer 2000). Precision can be measured in either phantoms, normal individuals or in patients with osteoporosis. Precision is optimum in phantoms, and will be less good in patients with osteoporosis than in normal people, because positioning is more problematic in the former. Precision can be calculated by making repeat BMD measurements in the same individual after repositioning (usually a minimum of 10, but preferably 20 individuals or patients), although the International Society for Clinical Densitometry recommends 100 repeat scans (ISCD Position Development Conference—Writing Group 2004). Departments performing bone densitometry should ideally calculate their own precision but this is not always feasible. Precision is optimised by using the minimum number of expert and highly motivated and well trained technical staff; it is not ideal to have a large number of staff who rotate through different departments and perform bone density scanning only infrequently.

Accuracy is how close the BMD measured by densitometry is to the actual calcium content of the bone (ash weight). The accuracy of DXA lies between 3 % and 8 %. The inaccuracies are related to marrow fat and DXA taking soft tissue as a reference (Tothill and Pye 1992; Blake et al. 1999; Blake et al. 2009). Although DXA measurements are affected by accuracy errors similar inaccuracies are present when other basic clinical measurements are made of blood pressure and body temperature (Blake et al. 2012).

Specificity is the ability of the measurement to discriminate between patients with and without fractures, and to measure small changes with time and/or treatment. A statistically significant change in BMD is calculated from the precision of the measurement technique. To reach a statistically significant change, the BMD has to increase or decrease by at least 2.77 times the precision error. This is termed the least significant change (LSC) and implies that changes in BMD of 3–4.5 % in the lumbar spine and total hip, and of 6–7.5 % in the femoral neck BMD (Gluer 1999; Bonnick et al. 2001) have to be present for change in BMD to be significant. Changes in bone density are generally small; even in the

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