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Setting up an empirical model of optical sensing to exploit the circular Bragg phenomenon displayed by chiral sculptured thin films (CSTFs), we considered a CSTF with and without a central twist defect of ??/2 rad. The circular Bragg phenomenon of the defect-free CSTF and the spectral hole in the copolarized reflectance spectrum of the CSTF with the twist defect were both found to be acutely sensitive to the refractive index of a fluid that infiltrates the void regions of the CSTF. These findings bode well for the deployment of CSTFs as optical sensors.