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The ultrasonographic evaluation of inferior vena cava (IVC) parameters, particularly the collapsibility index (CI), has been widely used in the assessment of the fluid responsiveness of critically ill patients, but the results are conflicting. In this study, we aimed to investigate the early change in CI after increased intravascular volume (IVV) induced by passive leg raising (PLR). A total of 145 healthy volunteers over 18 years old were enrolled between September and December in 2015. Before and 2 min after PLR, the maximum and minimum IVC diameters (maxIVC and minIVC) were measured by color Doppler ultrasonography, and the difference in CI (ΔCI) was calculated. The heart rate (HR) and noninvasive mean arterial pressure (MAP) were also monitored. We found that there was a significant increase in the mean maxIVC and minIVC values and a reduction in CI. Nevertheless, no significant differences in HR or MAP were observed before or 2 min after PLR. The baseline CI had no relationship with individual characteristics and a multiple linear regression analysis of the ΔCI and individual characteristics showed that age, baseline CI, and BMI were independent variables for ΔCI. In conclusion, IVC-CI measured by ultrasound is useful for the detection of early IVV change induced by 2 min PLR. However, its ability to detect the increased IVV value is influenced by age, BMI, and baseline CI. Moreover, only 50.3% of the subjects had an IVC-CI reduction of more than 10%, making IVC-CI of little value for clinical applications, due to its poor sensitivity.