An apparatus, and method of its use, to determine a characteristic of a fluid sample contained inside a closed container. The ratio of intensity for detected radiation (e.g., light), subsequent to its transmission through the container's walls and fluid in the container, at each of a reference wavelength and a measurement wavelength is compared to a predetermined value to make a determination of the fluid's characteristic. Desirably, the wavelengths are applied to the same location on the container to minimize a source of signal error. The reference wavelength is selected for its substantial lack of attenuation when transmitted through a known fluid composition. The measurement wavelength is selected for its predictive interaction (e.g., absorption) with the fluid.