Stiffness index identifies patients with osteoporotic fractures better than ultrasound velocity or attenuation alone. 
To compare a composite ultrasonometry variable, the stiffness index (SI), with its two component variables of speed of sound (SOS) and broadband ultrasound attenuation (BUA), in identifying post-menopausal women with low bone mineral density (BMD) and/or osteoporotic fracture.  A cross sectional sample of 1217 women (mean (S.D.) age 53.9 (9.7) years) was studied.  Risk factors for osteoporosis were assessed by detailed questionnaire and women with diseases, or those taking treatments known to affect bone metabolism were excluded.  Women were allocated to one of four groups: pre-menopausal women (n = 476), healthy post-menopausal women (n = 583), post-menopausal women with low BMD (n = 101), and post-menopausal women with osteoporotic fracture (n = 57).  An Achilles ultrasonometer was used to perform quantitative ultrasonometry (QUS) at the os calcis.  The SI.  calculated mathematically from SOS and BUA, was computed.  Analysis of receiver operating curves (ROC) between healthy post-menopausal women and post-menopausal women with low BMD but no fracture, showed that the area under the curve (AUC) for SI was significantly greater than that for BUA (P < 0.001) or SOS (P < 0.05).  For healthy post-menopausal women compared to women with fracture, the area AUC for SI was significantly greater than that for BUA (P < 0.05) or SOS (P < 0.001).  No significant difference was found for AUC between BUA and SOS.  QUS variables discriminated women with low density or fracture from healthy postmenopausal controls.  The SI was a significantly better indicator than BUA or SOS in this retrospective study.