Diagnostic agreement of two calcaneal ultrasound devices: the Sahara bone sonometer and the Achilles

Quantitative ultrasound for the assessment of skeletal status is an evolving method in the diagnosis of osteoporosis. In this cross-sectional study we investigated the diagnostic agreement between the Sahara bone sonometer and the Achilles+ with respect to broadband ultrasound attenuation (BUA), speed of sound (SOS) and stiffness/quantitative ultrasound index (QUI). 309 healthy females without diseases or medications known to influence bone metabolism (with the exception of oestrogen) were recruited at two participating centers (Erlangen and Berlin). 33% of subjects were taking oestrogens. There was no significant difference in BUA, SOS, and stiffness/QUI between oestrogen and non-oestrogen takers. In vivo precision (expressed as root mean square coefficient of variation) was calculated from two repeat measurements and analyzed in both centres. Mean values were 1.57% (BUA Achilles+), 3.64% (BUA Sahara), 0.35% (SOS Achilles+), 0.39% (SOS Sahara), 2.22% (stiffness Achilles+) and 3.04% (QUI Sahara). Between the two devices we observed a strong correlation for SOS (r=0.89, p<0.01) and stiffness/QUI (r=0.83, p<0.01), and a moderate correlation for BUA (r=0.68, p<0.01). All parameters were moderately negatively associated with age (r=-0.38 to -0.48; p<0.01 for all correlations). Kappa (kappa) scores used to report diagnostic agreement were calculated for tertiles and "equivalent T-scores". The tertiles divide the cohort on both scanners into the same number of subjects above and below a given T-score. Diagnostic agreement using tertiles was poor to moderate (kappa less than or equal to 0.51). Diagnostic agreement using equivalent T-score agreement, again, was poor to moderate for BUA but fair to good for SOS and stiffness/QUI (0.59 less than or equal to kappa less than or equal to 0.73). We conclude that diagnostic agreement between the two devices is at best comparable to the agreement of a dual X-ray absorptiometry measurement using the same densitometer at two different skeletal sites. It is therefore insufficient to compare directly two measurements of an individual patient on both ultrasound devices. Standardization of quantitative ultrasound is very much needed.